The diners clinked and tattled and the thick smell of food filled the air. Their moulded trays were between them, the remains of various unfussy foods in the different compartments: chickpeas, green beans, kesah bread and some kind of stew. The refectory strip lights gleamed in the battered metal cutlery.

'There was a fight or something last night.'

April looked up from her darning at the woman opposite her. Jean was older by four years, salt and pepper hair framing an oval face.

'Really? What happened?'

'I'm not sure. We heard the noises in the corridor. It was late, Glenn and I were in bed, we were woken by thumping and shouting. At first we thought it was some sort of game – you know, kids enjoying the first night of St Kilda's – but then Glenn looked out the spy hole, and saw goodness knows what.'

Jean paused, hands resting lightly on the stained plastic table-top, letting the pleasure of her outrage hang in the air.

'So what did you do?'

'Glenn phoned the police. They took long enough: by the time they got there, the hoolis had gone. They had trashed the corridor, though; smeared paint all over poor Wendy's door, slashed the carpet and the walls. Terrible mess. It will be weeks before it's sorted out. I tell you, this colony is going to the dogs.'

April shook her head.

'That's awful. Did they see anything on the security cameras? Anything to tell who it was? Did Glenn see anyone's faces?'

Jean shrugged. She was unimpressed at the practical response.

'They wouldn't do anything if they could. The police are just interested in making sure their response times are acceptable. Why would they do anything about a vandalism? And what that poor woman has been through already.'

April glanced around at the other people there, lost in their own worlds of conversations and food, at the queue that snaked around the serving area and from the tills. No one seemed to be listening.

'You mean about her son? Rheese?'

Jean narrowed her eyes.

'Yes. How did you know about that?'

'He's in the same classes as Harry. When he was expelled it was something of a scandal; there's never been an expulsion on the colony before. They hushed it up by saying he was ill, which I think he actually was, but that's not the reason he was ejected.'

Jean pushed her tray aside, and leaned closer.

'He is ill. And he was kicked out for arguing with the teachers over some stupid face mask or something. I've seen him and his buddies hanging around. They all wear them. I think last night's little fuss was a bunch of his mates, I don't know, making some kind of pointless protest.'

'Oh well. In which case, the police will be able to find them easily enough. If they can be bothered.' She looked at her watch. 'Oh Jean, I'm sorry. I need to go to a meeting in quarter of an hour.'

'I'd finished anyway. I'll come back with you.' She smiled. 'I hate the Tuesday lunch menu. If they had it every day I'd be half my weight. Come on.'

* * *

The evening rush hour. The lights hummed and flickered as the lift sunk down the tower. The people around her swayed silently, moving to let fellow passengers out at their destination floors, each in their own silent bubble. April watched the green numbers crawl up the display, and floor by floor the lift slowly drained, until she was the last traveller. The zero that climbed into the centre of the screen was in white, and when the doors slid open, she stepped into the empty lobby.

She pulled her shawl around her as she strode down the corridor. It was always colder down here than upstairs, but it seemed especially chill this evening, the heat seemingly being sucked down into the floor beneath her. The corridor lights were frigid white, and the bare concrete seemed to be breathing icy air.

There were still people around, shutting up their stores for the night. She smiled at a man she knew, and he nodded back as he hefted a sack of something into a cart. Some shops were still open, and she bought a few perishables – soy milk and kesah bread and oranges – from the place nearest the flat, and rounded the corner into the narrower corridor. She slowed slightly when she saw its occupants.

There was a group of a dozen or so kids, young adults really, teenagers about Harry's age. It was hard to tell given their masks and hoods and baggy, muted clothes, but she guessed most were boys. They were lounging under a faulty light, murmuring to each other, kicking the wall or staring at flickering, palm sized electronic devices. As she drew closer, they stopped talking, and some turned to gaze at her.

They were not threatening looks, just bored and curious, but the stories that she had heard and the sight of herself reflected perfectly in their empty chrome eyes made the hair rise on the back of her neck. She looked down and strode forwards, nervous for reasons that she couldn't really justify. She stared at the floor, at the litter of food wrappers and drink cartons, and as she walked between them, their eyes boring into her, she tried not to flinch.

Just as she left the group she heard one of them make a strange hissing, snarling noise. She gasped, and glanced at the source; but it was just someone slumped over a phone or computer. It must have been the machine. Trying to show no emotion, she strode down the corridor, right to the end, to the door that she and her family called home.

She touched the palm lock with her hand and the latch clicked open. The place was in darkness. The boys must both still be in school; she frowned, trying to remember whether Joel had a match today or not. Yes, I think, she thought.

She was still slightly spooked by the encounter with the kids – although, if she was going to be honest, 'encounter' was too strong a word – so that when she brushed the light panel, and turned to put her purchases into their tiny kitchen, she stopped with a start, her heart pounding wildly; and then she realised the human form that was sat hunched on the stool was Harry, her older son.

She shook her head, relieved and irritated in equal measure.

'What are you doing here with the lights off? You gave me a scare!'

He didn't say anything, immediately. He was drinking something fizzy in a plastic cup, some relic from his childhood. His back was to her, and he was staring through the window that looked out onto the plane towards the mountains.

He took a sip.

'There's a storm. I was watching it.'

Shrugging, she put the food into the little fridge. She turned to look at his profile, staring into the navy sky, at his father's nose and chin, at his long hair pushed back behind his ears. She breathed out, feeling the calm that he exuded, enjoying the sensation of being home again.

'Uh-huh. Mind if I join you?'

He nodded, expressionless. 'Sure.'

She pulled up another stool, plunged the flat back into darkness, and sat next to him.

There was indeed a storm, a vast tower of black cloud above the dusty mountains to the east. The purple lightning twitched and shivered around the peaks, and the setting sun threaded its rays through the crags like a crown. The two sat in companionable silence for a time, letting the vista soak into them.

'I came home early from school, today, Mum. I didn't feel so good.'

'I'm sorry to hear that. What kind of not so good?'

He shifted in the darkness, sipping his drink.

'Just kind of muzzy.'

She nodded.

'Do you want me to make an appointment with Doctor Hendrick?'

'No, I don't think so. It just feels like some kind of cold.'

'Ok.' She weighed up her options. 'You should probably...'

'Go to bed. Yeah, I know.'

She smiled, despite herself, and then looked around as the door banged open, and the lights blazed on.

'Hi mum! What are you doing in the dark... Oh.'

Joel stopped up short as he saw Harry. A heartbeat, and then the older boy stood, wordlessly, pushed his drink aside and left the room, eyes fixed on the floor.

'Night, mum.'

She looked between them for some clue.

'Have you two had another row? Oh, honestly...', but Harry was already in his room, the door quietly clicking closed behind him.

'What was all that about?', she asked Joel.

He looked at her, and then shrugged.

'It doesn't matter. Have you got some bread? I'm starving. We don't get fed properly after football, I tell you. We lost, by the way.'

'Yes, I brought some back... here. I wish you two wouldn't fight so much.'

He looked up at her as he slapped something chocolatey on the bread, and sighed.

'I'm twelve; he's sixteen. What are you expecting? Besides which, he's being a dick at the moment. I think you should stay out of it.'

'He seemed pretty reasonable now. Just a bit under the weather.'

Joel frowned.

'What kind of under the weather?'

'He didn't really say.'

Joel looked about to say something, and then changed his mind. 'I've got some homework to do for tomorrow, so I'm going to get on with it. Tell Dad not to retire just yet: unless we can get a decent goalie I don't see the talent scouts stopping by.'

She smiled at him.

'Oh dear, did it really go that badly?'

'Well I had a pretty good game, but our defence might as well have been on a different pitch for all the good they did. Never mind.... Do you know what a cryo-volcano is?'

She shook her head.

'No, me neither. Gotta find out for tomorrow. Rajeev said it was a dirty great big hole in the ground that chucks out water ice, but I think he might have been taking the piss.'

'Oh, please stop swearing so much, Joel. I don't know; I wasn't born here. The geography at home was all glaciers and scree and oxbow lakes.'

'What's a glacier?'

'Sort of a huge frozen river. They move really slowly and leave these amazing valleys.' She smiled, remembering a skiing holiday in Switzerland as a young girl, recalling the crunchy feel of the snow under her boots. 'There's nothing like that here, really.'

He nodded, thoughtfully.

'Well, I shall add them to my list of weird things to write about if Rajeev is right. Thanks mum! Night.'

'Night, Joel.'

He kissed her, and wandered to his room, carrying the sandwich. She followed him with her eyes, and thought, they are so different, those two. Joel the athlete, Harry the artist. The doer and the thinker. Maybe it's just puberty that separates them, and Joel will also become moody and taciturn in time, but somehow I doubt it.

Oh, she thought, how I hate this place sometimes. How lonely it is. How so far from home.

She cleaned up the mess that the boys had left, then walked into the tiny study, turned on the computer and looked for messages from Tomas. Nothing. He was on his last week of his shift on the ark, and he was probably only three or four light hours away now as the ship made its return orbit towards their little planet, and she was disappointed not to see anything from him. I hope you are well, my love, she thought. The boys miss you. So do I. She typed a quick note to let him know how Joel's match went, thought about mentioning that Harry was ill but then decided against it; it didn't seem that important.

She sat at the computer for an hour or so, working on turning the tide of forms and bills and requests and surveys that seemed to constantly wash into her inbox. As she did so, she felt herself fretting about the boys, asking the same question she had ever since they had been born. Did we do the right thing coming here? As ever, the answer was simply, I don't know. I hope so.

Maybe Harry has the right idea, she thought. Maybe I should just sit in the darkness and watch the storms. This planet has a wild, sterile beauty.

She turned off the light, darkened the computer, stared out of the study window. The lightning was still raging over the far peaks; the sun had completely set. The flashes lit the peaks and briefly revealed their jagged features.

It's funny being down here, she thought, down at the base of the tower. At the time, it made sense; my job was close by, and Tomas was mostly working on the stuff in the basement, and no one wanted apartments down here, so it was cheap. And now, it's almost entirely retail on the bottom three floors. If we wanted we could sell and move high up, maybe even get a bigger place than this. That butcher, what's his name? Devon. I think. He wants this place as a storage room. A meat locker.

She stared into the darkness, lost in her thoughts.

That's peculiar, she thought. It must be a trick of the light. It looks like there are people out there. She squinted into the shadows.

No, definitely people. They were moving fast, faster than they should have been able to in exosuits, lurching and bounding forwards, limbs flailing in the night. Oh goodness, she thought, no one should be out there now. There must have been some kind of accident.

She picked up the phone and called the night line, still staring at the figures outside. As they rushed towards the tower, one of the farm domes obscured her line of sight, and they disappeared from view.

'Night watch, hello?'

'Hello? I think there must be some kind of problem. I can see half a dozen people outside, in or around farm dome eight, moving towards the eastern entrance of tower one. They look like they are hurrying.'

'No personnel are assigned surface assignments on storm nights.' The operator sounded bored, dismissive.

'I understand that, that's why I'm calling. Has there been any accidents reported?'

'Nothing tonight, ma'am. Are you sure you have seen these individuals? Where are they now?'

'Beyond dome eight. I can't actually see them at the moment... No, wait, they are visible again. They are moving straight for the eastern entrance...'

They were getting closer now. It was still hard to make them out, but as she saw them hurry across the short distance from the dome to the door, she saw something that made her gasp: 'they aren't wearing suits!'

The response was as scathing as she was worried.

'It is not possible for humans to survive in the native atmosphere without a suit...'

'I know! I'm a terraforming biologist, that's why I'm calling, something must have happened.'

'If they are heading towards the door, they should be able to call for help when they get in.'

'Oh, for goodness sake... if you won't do anything, I will. I'll go to the door and look, and call you back.'

* * *

As she hurried through the corridors she was struck at how quiet it was. There was no sign of the kids she had passed earlier, or the shop workers. The only moving thing she passed was a cleaning robot that slowly hummed its way along a side way, its flashing red lights and spinning brushes causing shadows to dance around the darkened space.

As she got closer to the main entrance, she pulled her phone out and checked it, just in case. It seemed fine. She wondered how far the people would have got; at the speed they were moving, they would have been at the entrance for some time, a couple of minutes or so. She hoped they were OK; hoped that no one was seriously injured by being out in the poisonous air. Hoped that she wouldn't just find bodies. This is insane, she thought.

She could see the entrance hall now, huge and dark. The east door was the one that all the crawlers came and went from, so the hall was cavernous and cold, big enough to fit a dozen of the giant wheeled vehicles. Normally it was bustling with people and machines, but now it was deserted. It felt like an abandoned cathedral, with its huge glass airlock on one side and spherical lights suspended from the vaulted ceiling.

She hurried in, her steps echoing around her, glancing around for some sign of life. There was a stack of transport containers across the hall, the huge metal oblongs piled three or four high and she thought she saw some kind of movement in its shadows, but she dismissed it as another robot. Yes, a flashing light; it must be another cleaning machine. As she strode through the room, she saw that the lights above her lit up, leaving a trail of glowing globes that mirrored her own path, brushing the darkness to the walls of the hall. It gave her cause to think: why are there no lights on by the door? They must have not made it. She quickened her step.

She wasn't heading to the main vehicle airlock, but the smaller personal one to the right. It was also glass doored, so you could see into its chamber and beyond onto the planet's surface; but the lights that followed her reflected off the glass, and gradually she was staring at herself as she hurried towards it. There was no sign of life.

When she was a dozen footsteps away, the external floodlights clicked on, and a great pool of light saturated the ground outside, revealing it again. The rough road that stretched away from the colony was picked out by tiny, soft yellow lights. She could see the boulders strewn along the ground, and the tracks of the crawlers in the soft dust.

She walked right up to the airlock, and pressed both palms against it, staring as hard as she could into the darkness beyond.

Nothing.

It was hard to see because of the reflections of the room behind her, but her eyes were straining for movement, anything to indicate that there was life out there. She squinted, and then saw a slow shift of shadow that could have been, what, an arm? She frowned and looked more and then realised something that filled her stomach with ice.

That movement was reflected in the glass.

She turned around, and stared at its source: the containers. Maybe that hadn't been a robot.

Suddenly nervous, she started walking towards them, grateful for the silent assistance provided by the lights.

'Hello? Are you OK? Is anyone hurt?'

There was no reply, but she wasn't expecting any; anybody in need of help would have called as soon as she had entered the hall.

Nevertheless, she could see the arm. Its owner was behind a container at the end, separate from the main stack; only the elbow protruded. She wondered how she had been unable to see it when she had entered. It shifted, slowly, lazily. What in God's name is going on?, she wondered.

She started walking in an arc, so that she was moving around the container rather than towards it, staying at the distance where her caution and conscience cancelled out. As she got further, she saw a pair of legs, leaning against the great metal oblong, a jacket, a head...

She stopped short. It was the kids from the corridor earlier.

They were all there, sitting and leaning on the containers. Some were looking at her, but most of them couldn't be bothered with even that. They just stared at the ground or the walls.

Not sure what to do next, she started approaching them with as much gravitas as she could.

'Hello?', she called, 'Hello? I'm looking for some people. I think they might have had some kind of accident, might be hurt. I think they came through the airlock recently, through here. Have you seen them?'

A couple shrugged, looked away. Only one of them continued to stare at her, motionless. She thought that this one might be a girl.

'Hello?,' she called, irritated. 'This might be an emergency! Have you seen anything?'

'No,' said one of them, muffled by the dust mask. 'We'nt seen shit. Nuthin.' He spoke slowly and with effort. Those damn masks, she thought. What a ridiculous fashion.

She shook her head, exasperated.

'Ok. Fine. I'll keep looking then.'

She wanted to say more, but she couldn't think of anything that didn't sound shrill, so she simply turned and walked away. She had only taken half a dozen steps when she heard a voice behind her.

'Mrs Tyler? Mrs Tyler?'

The voice was a girl's, and despite the mask, April thought she recognised it. The girl who had been staring was following her, not moving with any great speed but clearly trying to catch up. She stopped and waited for her.

Close up, she seemed exhausted. Despite the mask and hood she was wearing, April could see the pale, drawn skin and bags under her eyes. No wonder they lounged so much, this lot.

'What is it?'

The girl's contact lenses were tiny, perfect mirrors that filled her eyes. It was impossible to see what she was looking at, but April felt her gaze upon her.

'Mrs Tyler? It is you, isn't it?'

Her voice sounded laboured, exhausted.

'Yes, it is. What is it?'

'We didn't see anything, Mrs Tyler. We've not been here that long. If someone had come through recently we'd have seen them, but we've only been here five minutes or so.'

April nodded. 'Ok, thanks. I appreciate it. Say, do I know you...?', but the girl had already started walking back to her friends, and didn't seem interested in further conversation.

With a shrug, she set off home.

* * *

That night she slept badly, strange thoughts and – eventually – dreams lashing around her mind. She woke late, still twisting the questions in her mental fingers, unsatisfied with what little sleep she had had, but also strangely awake. She swung around her legs, and sat up on the edge of her bed, listening to the morning, trying to decide what to do. Nothing made sense.

Outside her bedroom she could hear the sounds of the boys getting ready for school. Judging by the wordless noises, they were still not talking. She heard the front door open, and she stretched and yawned. Oh, boys, she thought. One day Tomas and I will be gone, properly gone, and who else will you have then? She tried to remember how much she had argued with her sisters.

'What is it?'

That was Harry's voice. She couldn't hear the response, but he continued, 'there's nothing to talk about.'

Joel must have walked up, because she heard him this time.

'Yes there is, and you know it. Come on. Mum has gone to work, now is fine.'

She frowned, and wondered what to do. Cowardice and nosiness overtook her, and she remained silent.

'Joel, I need to get to school...'

'I know you took the blood oath. Yesterday. You shouldn't have done it. It was fucking stupid.'

Silence for a beat. When the older boy replied, he sounded completely different, less dismissive, more afraid.

'What do you want?'

'I've been doing some research. Talking to some people. I'm worried about you, Harry. What did you do it for?'

'I... it doesn't matter.'

'No, I suppose it probably doesn't, now, does it? Well, if it was for that girl, she wasn't worth it.'

She could almost hear Harry squirm, but Joel was merciless.

'I've seen her, Harry. Seen her without that stupid mask that she wears all the time. What's her name? Leena? She might have been pretty once, but she's fucked up now.'

'How? I don't believe you. You little...'

'I've seen her because her younger brother is on the team. I was over there only last week; she came out of the bathroom just as I was going in. She wears that mask because she's covered in sores, right? And Jesus... her eyes. Have you seen them? She wears a lot of make-up, they all do, all of that lot, even the guys, but it's not enough. You can see it when you look. They are ill. All twisted up, and getting more twisted.'

Silence.

Leena. That was the girl who had spoken to her last night. Harry has known her for years; she remembered going to her eighth birthday party. Another native born child, another human who had known nothing but these towers and domes and empty, rocky plain.

Joel continued, the words hard and remorseless, 'that's why he was expelled, wasn't it? It got so bad he started to wear his mask in lessons, and the teachers kicked him out.'

'Yeah. Maybe. Look, you've said your piece, what do you want?'

'I want you to get to a doctor. Mum doesn't have to know...'

'So this is brotherly love?'

'Stop being a dick! Have you listened to nothing I've said? You're ill!'

'I am not! Well, yes, I am. But you don't know everything.'

'Yes, I do! You were sitting in the dark last night to watch them, weren't you? Watch them go out? I know everything! Just because I'm younger than you doesn't make me stupider.'

They were shouting now, the physical anger leaking into both their voices. She couldn't listen any longer. She stood, opened her bedroom door, and walked into the living room.

Both tried to hide the shock on their faces. They were sitting opposite each other, Joel on a sofa, Harry on a chair; both were in school uniform, bags at their feet. They stood as she came in, trying to get away, but she spoke a single sentence before they could move.

'I saw them too.'

They froze.

'I thought you were out...'

'I'm not. I'm here.'

A horrible, horrible silence.

'Look, I didn't hear it all,' she lied, 'but I heard that you are ill, Harry. And I saw those kids outside. I was looking out of the window, saw something and thought there had been some kind of accident. I went out to the entrance hall, to see who it was. All I found was them. You know them, don't you Harry?'

She knew the boys would be racking their brains, trying to remember what they had shouted, what they had whispered.

'Yeah, I do, Mum.'

'OK. I want to get you to Dr Hendrick. You can miss school this morning; I'll make the appointment now. Joel, you get going as normal.'

He nodded, obviously relieved to be leaving.

'Oh, and Joel? Not a word to anyone, yeah?'

'OK, mum.'

They were silent until the door closed. Harry looked at her, and she could see pain in his eyes.

'Mum, I...'

'Don't worry. Look: let's talk more when we understand what this is. Maybe we can just forget it all. Maybe this can be just between the three of us.'

'OK.'

* * *

Dr David Hendrick was an old friend of her and Tomas. He was younger than them, but had a completely white head of hair that made him seem older than his years, and in calmer times she liked to tease him about the gravitas that gave him. They were sitting in his small office, the walls covered with posters that lectured on hand-washing and breast-feeding and exercise. He was gazing at a screen, frowning. Finally he looked up, and spoke.

'It's a virus, April. Nothing from the database, though. Totally new.'

'Nothing? Really? It's totally novel?'

Harry sat on the couch, staring at his phone, bandage around his upper arm. She wondered if he was even listening.

'No, nothing. The system wants me to report it, send it to Central for their computers to look at it. What do you want me to do?'

'I... can it wait? I need to think.'

'Of course. Hang on...', he tapped the screen, 'I've told it we are going to do further analysis. That's broadly true.'

'Thanks. I appreciate it.'

He nodded, and then looked at the boy, 'Harry?'

He didn't reply; just lifted his head from the tiny rectangle of light in his hands and looked at them.

Hendrick was measured, bland. 'So how do you think you picked up this little bug? Doing anything unusual recently?'

A shrug. 'I just... got this thing.'

She felt her hackles rise, and snapped, 'for goodness sake, Harry. Now is not the time...', and then she remembered. 'What's this blood oath?'

If she hadn't been worried, so very worried, she would have felt sorry. When she said that, as the last secret was stripped out, she could see something in him crumble. Suddenly he looked like the child that she remembered, a toddler being caught with something forbidden.

'I'm sorry, Mum'.

'Don't be. Just tell us what happened. This is some kind of blood exchange, isn't it? Cutting thumbs and rubbing them together, some brotherhood ritual or something...'

He didn't need to answer.

She turned back to Hendrick. 'There you go. It's blood borne. They all have it. Sores around the mouth, breathlessness, something about the eyes. God knows what else. Have you seen anyone else, any other clues?'

Hendrick nodded, slowly. 'One other. I don't do so many people from your tower, but I saw a lad with those symptoms. Obviously I can't say who...'

'Rheese Ifans.' She said, immediately.

Hendrick raised an eyebrow.

'I think he's the infection source.'

'You put me in a difficult position, April...', said Hendrick, uncomfortably.

'Yes, I'm sorry. Look, if this is a novel pathogen, it's almost certainly notifiable, so it will be public eventually. When this patient came in, you didn't see the virus?'

'No, he wouldn't let me do a blood scan. After an external examination I just assumed he'd picked up something fungal, and gave him appropriate medication.'

She nodded. It made sense. And something else clicked in to place, too.

'Can I see the screen, please, David?'

Hendrick shrugged. 'Of course. You probably see more viruses in your work than I do.'

She smiled. 'Yes, I probably do. Thank you.'

The system was similar to the one she used at work. She hunted around until she found the tools she needed, configured and launched them. There was a brief silence as the software ran, and then a soft 'ping'. She stared at the screen, grimly: her worst suspicions had been confirmed.

Hendrick shifted next to her so he could see. She pointed at a string of letters.

'Know what that is?'

He shook his head. 'It's been a while since I did this...'

'Fine. There is a virus here, with two distinct jobs. That's the primary. It's designed to patch the host's DNA, make more copies of itself. The secondary, here, invades the immune system and changes it, to stop it rejecting the modified DNA introduced by the first.'

Realisation dawned on Kendrick's face. 'But then, these are...'

'Yes, exactly: nucleotide adjustment agents, NAAs. Viruses designed to fundamentally change organisms; what I use in terraforming. These are man made, or at least they were in their original form. They might have mutated since then.'

'What's the payload?'

She sat back in the chair. 'Couldn't tell for sure without taking it to the lab, but based on what I've seen of the infected, it lets you breathe the air outside.'

They were silent for a second. Hendrick picked up a pen and started twirling it over his fingers as he thought.

'Well, there are anti-virals for NA agents. Standard ones will do, assuming it's not mutated too much, as you say. They will probably work for Harry, because he's only just picked it up; but anyone who's had it for more than a week or so will have had their genes substantially altered, so simply killing the virus won't be enough. Although it will probably alleviate some of the symptoms: I would imagine a lot of what they are going through is an immune reaction.'

She nodded. 'Right. Most NAAs take about a month to do their stuff, although I'm not much the expert on humans. I mostly work on fish... Never thought I'd see my son infected with one. Be grateful you are not a fish, Harry.'

Harry looked up and smiled weakly, while Hendrick tapped away on the computer.

'Well, you were right. This is notifiable; and I can't really delay sending it to Central any more. I've noted what we think it is; and, off the back of that I've prescribed a course of the appropriate anti-virals. Pick them up on your way down. Harry, you get to stay at home for the rest of the day; but come back tomorrow and I'll do another test and we'll make sure the virus count is going down. I expect you'll be off school for a week or so.'

She smiled a relieved smile, and stood.

'Thank you, David. You've been a huge help.'

He stood, too. 'Not a problem. Let's make sure we meet again in happier circumstances... So, if you are not, who is the expert on human NAA here?', he asked.

Her smile became wry. 'Dr Euan Iffans.'

'Ah.'

'Yes.'

Hendrick scratched his chin. 'Well, I'll have to call him: tell him his boy has been in the medicine cabinet. We'll try and work out a strategy for the others, too. Anyway, my regards to Tomas.'

'And my love to Eileen.'

* * *

They took some time getting home. The two of them went to one of the communal gardens on the top of the tower and ate ice creams, and laughed and talked about nothing. She felt relieved, and closer to Harry than she had been in months.

However, her relief drained the moment they reached the corridor that led to their apartment.

'What on Earth...?'

The hall had been wrecked. The plastic floor had been slashed, broken glass was strewn everywhere from smashed light fittings. There was food smeared across the walls, and across their door this was sprayed in spiky red letters:

we want r own

She put her hand to her mouth.

'What does that mean? Harry?'

He shook his head.

'I don't know.'

She put her hand on the palm lock, and the dented door grated open. On the other side, another surprise: Joel was standing there with a hockey stick, wild eyed and ready to attack. When she stepped in he actually started to swing, but stopped when she shouted, 'Joel! It's me!'

He lowered it. He was pale and shaking.

'What's happened? Why are you home? Are you OK?'

'I'm so glad to see you.'

She hugged him, unable to believe the state he was in. Harry wordlessly went into the kitchen and returned with three mugs of hot chocolate. Joel was silent until he had taken a couple of sips.

'I don't know what happened today, but they know. They know that you went to the doctor. They realise they've run out of time, that someone's blown the lid on them. They are proper fucking crazy. They had a go at me, but I ran away; I came here because I couldn't think of anywhere better. I guess they know where we live. What were they doing out there?'

'Trashing the corridor. They sprayed something, "we want our own", whatever that means.'

'They are coming for me.'

They both turned to look at Harry, who was perched on the edge of the sofa, his head down, his hair partly obscuring his face.

'What do you mean?'

'His plan was always to take his group outside once they were ready. Everyone. They included me in that list. Now they know that their secret is gone, they'll be doing that.'

'When?'

'I don't know... but I'd guess tonight.'

She rubbed her eyes and forehead with the heels of her hands, suddenly trying to fight back tears. She didn't want to have to deal with this, not on top of everything else.

'Mum...?'

When you are an adult, there is no one else, she thought. Just you and the world. No one to pick you up and wipe your nose and kiss you better. You do it yourself or it doesn't happen.

She looked up at them.

'It's ok. I'm just a bit tired. Harry, take your pills. I'm going to make some calls.'

* * *

As she expected, the watch were dismissive: she knew that she was on file as the crazy woman who had made the hoax call last night. Jean was far more sympathetic, however. Within the hour she and her husband Glenn had come down from their floor, and were sitting in their living room, drinking coffee and planning.

Glenn was a short, gruff, heavyset guy who worked as a gardener. She had always thought him aloof, but now she was actually spending time with him, she realised he was just painfully shy. Jean, in sharp contrast, was her usual chatty self.

'So what do you think we should do?', she asked April, holding a cup of benzu tea.

'I don't want to be forced out of my home by a bunch of kids. I spoke to Dr Hendrick again, and he's going to make a referral to the police, try and get them to take us a bit more seriously. I'm not sure how long that will take, though. Until then, I want to make this place a bit more secure. But I'm not sure how. Anyone got any ideas?'

'They can go outside, you say?', asked Glenn.

'Yes. Do you think they'll come through the windows?'

He shook his head. 'No. They are designed to survive a small meteor strike. Kids won't be able to break 'em. It's your front door you need to worry about.'

'Ok. How can we secure that?'

He scratched his chin. 'Not sure, yet. Although I have some other ideas. No neighbours here?And all the shops will be shut in the next hour or so?'

'Yes.'

'Good. You lad,' he pointed at Joel, 'you can help me get some things from the nearest dome. The tunnel entrance is close, it'll only take twenty minutes or so.'

And off they went.

They were back sooner than Glenn had promised, although it was already darkening outside and most of the shops had packed up; there were very few people wandering in the main arcades, and none in their corridor. April wondered if they were avoiding the vandalism.

Glenn and Joel set to work as soon as they had returned. They closed the heavy fire doors at the end of the corridor, and chained and padlocked them closed, stringing the chain through the handles.

'Let's hope there isn't a fire,' said Joel. Glenn grimaced and did not reply.

The second pair of fire doors were just outside their flat. These were poorly maintained and one of them refused to swing out of its niche in the wall, leaving the corridor only half closed. Joel and Glenn both pulled as hard as they could, but the thing refused to budge.

'Blast it,' grunted Glenn. 'When was this last inspected? Don't know why it's stuck. It would have been nice...', and then the lights went out.

April, standing in the door of the flat, gasped, and heard Jean curse behind her. It was pitch dark in the corridor. 'Guys,' she said softly, 'what's happened? Did you do that?'

In answer to her, the closed fire doors started rattling.

'I think you two should come in,' she said. 'I'm just here.' She pulled out her phone, and filled the area with a pale light. She could see Joel was staring at the door, obviously spooked, but Glenn ignored them and carefully and methodically packed up. 'That should hold 'em, no point panicking.'

The doors were being shaken furiously now, the noise echoing around the corridor and inside the dark flat. Joel and Glenn came into the flat, and April dragged the door closed, having to use brute force now that the power was no longer there to run the motors. She touched the palm pad and it locked, thankful for the emergency battery inside the lock. She joined the rest of them sitting in the living room, the only light coming from the last faint glow of sunset through the window.

She shivered.

'How did they cut the lights?'

Harry replied, quietly.

'Luba's mum is in maintenance. He steals stuff from her sometimes. Remote controls, access keys, that kind of thing. Everything in the complex can be turned off remotely if you have the right codes and equipment.'

She remembered the dark entrance hall, illuminated only by her own presence. That made sense, now.

'It's cold,' said Jean. 'Can't you put the heating on?'

'I'll look,' replied Joel. 'I think I have a torch in my room... hang on.'

They sat in the gloom, listening to the racket outside, saying nothing. The rattling abruptly stopped.

April frowned. 'Do you think...?'

'No,' said Harry.

The next noise, when it came, made them all jump: a massive thud that shook the closed fire doors and rattled their chains, followed by another, and another, regular and slow, like the footsteps of some massive beast.

For the first time that evening, worry crossed Glenn's face. 'They must be using some kind of battering ram', he muttered.

Jean jumped to her feet, looking panicky. 'Then we need to do something, make sure they can't get through this door, don't we, Glenn?'

'That might not be a bad idea, Jean. Lad, where are you with that light?'

A blaze of light, and Joel appeared holding a small torch. It was only a little, but seemed brilliantly bright in the dark room. 'Here I am, Glenn. Can't get the heating working, sorry Jean. They must have cut the power to the pump.'

They could all hear the change in timbre of the banging; something was giving, some component of the fire doors slowly wearing down under the attack.

'Never mind that now,' replied April. 'We need to get something across this door. Can you move that bookshelf?'

Harry stood and started pulling books from the top shelves, Joel joined him.

'Let's try now...', he said to his younger brother, and the two started inching it across the room.

As this happened, Jean was getting more and more frantic. 'Quickly Glenn, darling! We don't have much time! Please, please...'

'Hush now, it'll be OK. There, like that.'

The boys, Glenn and April all pushed the heavy wood bookcase over the door, working silently, urgently. It was cold enough now that their breath started to steam in the torchlight, but despite it, she could see the sweat on their foreheads.

As they moved the bookcase into place there was a crash in the corridor outside. Jean screamed.

They ignored her, moving as many things as they could to support the bookshelf, to keep it in place: a pair of chairs, a table, the exercise bike, books, anything to give it mass. As they worked, there was another, louder, crash and they heard a ragged, snarling howl of fury and pleasure.

'Hey, Harry, dude. I'm sorry I called you a dick.'

April could hear the smile in Harry's voice.

'No worries. I'm sorry for being one.'

'Have this, bro. I'm not going down without a fight. Neither are you.'

'Thanks.'

The screeching of tormented metal filled their ears as something sharp was dragged across the apartment door, followed by a savage clang. Jean started crying. 'What do you want?', she shouted. There was no response.

There was a grating, grunting, straining noise, the sound of muscles pitched against steel, and then a dull clunk and the unmistakable sound of the door being dragged open. They all jumped back as their barricade began shaking. Glenn glanced at April, and then ran to his wife, who had lost all self control and was sobbing in the corner. The boys moved to brace themselves against the barrier, but April waved them away. 'No, it's too dangerous. Stay back. Joel, pass me your torch; do you have any others?'

'Yeah, here. It's a bit rubbish, though.'

'Ok. I want you to point it at the place I point it. Both of you, stay behind me. I'll handle this. I don't want you getting hurt.'

She was amazed to hear her own words, cool and crisp. She could almost see herself from outside her head, standing there, torch in one hand, facing the untidy pile of furniture; she saw her boys crouched down behind, wearing hockey masks and holding bats. She closed her eyes and inhaled.

Then, with a splinter of wood, a crowbar tip appeared through the bookshelf. The whole construct shuddered and heaved as the forces outside smashed themselves against it. She took a step back, fearful of the furniture falling on her, and the anger beyond hissed and growled, frustrated and exhilarated.

The breach was enlarged by hands ripping at the wood, howls of triumphant wrath streaming through the apartment, scraping fingers pulling away the backing, hissing filling the darkness.

She stiffened as she saw the hole grown; promised herself she wouldn't be like Jean who was beyond comfort behind her; hoped that her boys would be OK.

A face pushed itself through the gap, no longer masked, bare and base. It snarled at them, and she saw the wide irises and the flaps hanging from the lips, the organic filters grown from the virus. It disappeared just as quickly, the thudding and wrenching resuming, and she grasped the things she was holding like relics.

And then, with a crash, it was over; their barricade was thrust aside: her pursuers were standing opposite her, the wreckage of her flat between them, silence feeding out, nothing to say.

It was Jean who broke it.

'Leave us alone! What do you want?'

The figure at the front replied, hissing rather than speaking.

'He is one of us. Ask him.'

April flicked the torch up, saw them flinch. She couldn't make out forms, but she saw lumps where there shouldn't be, shadows dancing across their faces. She stood still, and asked the simple question: 'Why?'

Their leader snarled back.

'He took the vow. We are leaving.'

'What are you saying? Where are you going? There is nothing living out there. Nothing can go outside, except for you. The storms have started, but it will take a decade before there is anything other than puddles... you can breathe, but how will you eat? How will you drink?'

They swayed, but were silent.

'You have taken something that you don't understand. Please, please, let me help you...'

He took a step forward, hands on the chair, standing over the wreckage of the bookshelf.

'No, you don't understand. You aren't from here. We are the children of this planet, it is ours. We belong. You don't.'

'I don't...'

'You have never stood outside on the road that leads to the complex, seen the towers from outside, have you? Even with a suit. We have seen it, with our own eyes, breathing the air. No one else has. No one. This is our planet!'

She spoke carefully, slowly. 'You are correct: I wasn't born here. This is your planet. That plain out there, that ends in the mountains and the ravines and the craters, that is your home; beyond it, to the poles and the equator. This is yours, and in three decades you'll rule it, and I'll sit in some little room listening to you on the radio. I'm not stopping that: I can't.'

'Good...' and he started to pull himself over the chair, into the room. She shone the torch at his face again, and this time held it there, stopping him dead.

His mouth and nostrils were covered in strands of mucus and skin; his nose had widened and flattened. The irises were huge, and great black shards thrust themselves from the pupils into the whites. His skin was raw and flecked with sores; he flung up his bleeding hands to shield himself from the light.

'The virus is making you photosensitive.'

'I know! Stop treating me like a child,' he wheezed, trying to push his way past the pile of stuff in his way, but still keep his eyes out of the light blazing from the torches.

'So stop acting like one.' Even to her ears her reply lacked conviction. He kept forcing himself forward.

'You're Rheese, aren't you? I work with your father...'

He hissed through the mess that had been his mouth, the flaps of skin dancing in the torch light.

'My father knows nothing!'

She sighed, despite it all. 'No, you're right, he doesn't; he really doesn't. That's what it means to be an adult. There are no easy answers, no simple fixes, no black and white. You can live in a child's world where you can solve problems with simple, dramatic gestures, or you can join the rest of us who know nothing.'

'But you...'

'But I what? Look, there are more of you than there are us. If you want to start your new world like this, by attacking a woman and her children, by taking her son, you can. I can't stop you. If you think that's how it should begin, then here we are.'

He stood there, panting, the craters in his face glistening in the light, the pink snot drooling from his nose and open mouth. She could see that his hair had fallen out and that great patches of his head were visible, all the the texture of burned meat. He was leaning on the chair, panting wetly, the exertions of the last quarter of an hour too much for his developing lungs. Beyond him shifted the others, all watching, waiting for the situation to resolve itself. All around was silence, punctured only by his tattered breathing.

She walked up to him, slowly, carefully. As she got closer, she could smell the stench of them, ammonia and decay and something else she couldn't place, felt the nausea rise in her and forced it back.

'I'm not your enemy, Rheese. Please don't be mine.'

He looked at her, met her eyes for the briefest of seconds, and then saw the other thing that she was carrying.

It was a tiny make-up mirror, the size of her hand, something she had been carrying all day; now she was holding it up so that it was at head height, facing him, and just for a second he stared at his own broken face. He saw the blood-laced phlegm dripping between his teeth, the sores all over his face and neck, the filthy mess that was his mouth. He hissed, and flecks of fluid danced in the torch beam.

'Rheese, this is killing you. Please. I can help you. I understand these viruses...'

He shook his head, but she could see that suddenly there was water in his eyes and on his cheeks that was not from the infection. He was panting even harder now, the air bubbling in his trachea, fingers gripping his support.

He coughed and the blood splashed onto the floor; and he looked up at her, and this time she saw only fear.

'We can't live here like this...', he panted. 'We need to get out...'

She spoke gently, quietly.

'Yes, you can. You can stay down here, on the ground floor, away from everyone else. We'll work something out. And these things can't be totally reversed, but most of it can be put back. There will be another virus designed to replace the changed DNA with original human stuff. What you've got looks like something that isn't ready, a prototype. Who knows? Maybe we can fix it so you can breathe outside but look a bit more human. I understand these things, I can help you.'

And finally he was just an exhausted, miserable child, the fury washed from him, in need of help in a vast unkind world.

She reached up, and gently pulled his ravaged face to her shoulder, and held him while he sobbed.

* * *

Tomas was tired, and his head hurt, and he wanted, more than anything else, a bed and a glass of water.

Normally, at four in the morning, the corridors would be deserted; but this morning there were people everywhere, milling and chatting. What's going on?, he thought. Is there some kind of festival?

The transport had left the ark a week early, so he was home far earlier than April had been expecting; he hadn't had a time to message her to tell her, and he was looking forward to her surprise when she saw him. At the moment, though, his bones were aching from the landing, and the jet lag meant that his body clock was hours out, and his eyes had that familiar dry, scratchy feel from being awake far longer than they wanted to be.

The crowds deepened as he got closer to the flat; and suddenly, instead of people in dressing gowns there were medics in scrubs, people wearing masks, police. He found his way blocked by a watchman; but someone appeared from the crowd, a masked man with a shock of white hair, who took his arm and guided him through. It took him a moment to realise who it was.

'Tomas. It's David.'

'David? David Hendrick?'

'Yes... this way, Tomas, please. I'll take you to your flat.'

'Thank you. Was there an accident? Was someone hurt?'

'Just come this way, please.'

He walked through the sea of people and robots. He ducked under police tape, was given a mask and gloves and a coat to wear, had his name taken by a woman with a clipboard. No one would look him in the eye, no one would answer his questions. Still disorientated by the flight, he wondered if this was some kind of dream.

His corridor had great white plastic sheets blocking it that he had to duck past, once, twice, and then through the devastation of what once must have been fire doors. Temporary lights were standing everywhere, spraying hard shadows over the canvas that covered every surface. The door to the flat was open, and a pair of medics were coming out, masked and gloved, talking about something he didn't understand. He just stared, totally unable to comprehend.

Hendrick took his arm and pushed him forward again.

'In here, please. Yes, that's right...'

His flat was unrecognisable. Everywhere he looked there were people, and mess, and more white covers and spot lights. He could see forms lying on the floor, tubes and pipes joining them to monitors, and in the centre of the room, still in her brown work dress...

'Oh, darling! You are home!'

She flung her arms around him, her mask scratchy on his neck.

He cautiously returned April's hug, not quite sure how to respond; she released him and gazed into his eyes.

'What's happened...?'

'Oh darling, I'm so glad to see you,' she smiled, her eyes shining. 'I tried to message you, but you must not have gotten it...' and then her eyes tracked to his head. 'What have you done to your hair?'

He smiled ruefully, and put his hand through the blond stubble.

'It was kind of a bet on the ark; I found these grey hairs, and I thought, now is the time to be a baldy, now is the time to shave it all off, so I did. I didn't tell you, because I didn't know how to say it...'

He tailed off, because to his bewilderment April was laughing harder than he had ever seen her laugh, doubled up, gasping for breath, tears streaming down her cheeks.

'What is it? What has happened? Do I look stupid? What happened here? April...? David?'

She tried to stop, resumed; and then, panting for air, straightened up, seemingly oblivious of the bizarre clinical scene around her.

'Oh Tomas, I've missed you, so much. We are in quarantine, but it will be ok. The anti-virals are working, and we've been inoculated... We have some house guests, rather a lot actually, and we've made some new friends. And oh... Don't worry about your hair. Wait until I tell you about what your son has done. Remember how we were worried about him getting stupid tattoos?'

And then she pulled her mask off and kissed him on the cheek.

'Come and see your boys.'

And, at that point, Joel came hurtling out of his room, ignoring the doctors and everything else, flung his arms around his father; and a moment later, Harry walked in too, mask on, long black hair tied back, and he embraced them all.

And they were all together.