Sarina's eyes ache with exhaustion and yet it seems she's no closer to the answers she craves. The lab is dark, her workspace brightened only by her computer screen and the faint glow of the mutagen sample beneath her microscope.

She doesn't understand, but she knows she will. The mutagen reacts with organic matter the same way regular mutagen does, but what stumps her is what it doesn't do. Regular mutagen, when coming in contact with two or more types of organic matter, splits the different cells apart and then knits them together again in a new way. This strange white mutagen completely ignores them, leaving the cells intact and unmutated.

Sarina has already pored over her TCRI file, specifically in terms of her mutation. Her fusion with inorganic matter is not unheard of; Donatello has entertained her with the tale of his mutated spy-roach before. But even then, this white mutagen does not react.

She groans and slams her head against the table. Think, Sarina, think! What else? She pictures the beast; cold, fast, ferocious. Destiny said it felt like her attacks didn't always land how they should have. Is there any chance that—

Sarina stops when she hears scuttling, looking up from the table just as a cockroach scurries across. In a flash, she grasps an Erlenmeyer flask and brings it down over top of the bug, trapping it inside a transparent prison. Slowly, her bright eyes drag to the tiny vial of mutagen in its holder.

She flips the flask right side up. The bug scuttles around, trying and failing to climb the steep triangular sides of the flask, and she brings herself down to eye level.

"I apologize, little one, but this is for science," she murmurs. "I am sure you will live."

She works quickly, locating a battery and a cable clamp. With some quick work, she connects the clamp to the end of the battery and lightly touches her finger to the other end of the cord. A pleasant buzz rushes through her fingertip. Perfect.

She will have to make this precise. She lets the cord dangle as she carefully feeds it into the narrow neck of the Erlenmeyer flask. The cockroach keeps running in circles as the electrified end draws near. She prepares the white mutagen with her other hand, hovering it over the opening.

Just as the cockroach touches the end of the cord, she pours a dollop of mutagen into the flask. There's a burst of light, enough to make her turn her face away, as she yanks the corded battery away. She waits for the light to die down, faintly aware of the electric buzz in the air, the sound of a neon sign humming.

She looks back and her eyes flash rapidly. The cockroach keeps running around the flask, its once brown body now glowing bright white, flickering and sparking like a livewire, an endless electrical spark.

"Oh my Darwin," she breathes. She looks at the bit of white mutagen still resting in her flask and quivers a little, inhaling sharply. "Ohhhh my Darwin."

The laboratory door creaks open and her head whips to it, eyes wide. Donatello appears, looking groggy as he stifles a yawn with his hand.

"Sarina? Are you...still..." He trails off, noticing the bright electric bug still trapped in the flask. "What is that?"

"Donatello! I finally have made progress! I now believe that I understand what the mutagen does," she says. "How is Raphael?"

"Well, that's why I'm here," he says. "He's getting feverish, but his whole arm is still cold to the touch. He hasn't woken up either."

"That...that makes no sense," she whispers. "We cleaned and debrided the wound. Infection should not be setting in and certainly not this soon."

"I know, and he should have at least been responsive by now. He has all the symptoms of an infection everywhere else. It's like, whatever this is, his body can't kill it off."

"And neither can we."

Donatello's expression falls. "I had Leo try using the healing hands. No effect."

Sarina curls her fingers into her palm, her thumb and pointer knuckle tapping against her chin as she starts to pace again, and he watches her as she starts to mumble to herself.

"Sarina?" he asks. She stops, looking back at him, and he wrings his hands. "Is this something you want to explain more than once?"

She shakes her head. "I do not think that would be preferable."

"I'll gather the team."

----------

It takes a bit, but soon Sarina stands before a bunch of groggy young adults. Destiny yawns and leans against Leonardo, still bundled in Casey's hoodie, while Leonardo does his best to look alert despite the dark circles developing under his eyes. On top of his exhaustion, he seems frustrated, gaze darting to his hands as he flexes them into fists. Michelangelo sways a little in place, eyes half closed, and Casey tries to blink away the endless itch plaguing him.

"I am sorry to wake you. I know the hour is early, but I felt this was needed sooner rather than later," Sarina explains. She produces the flask with the electric roach inside, holding it up. "I have discovered what makes the white mutagen unique."

Michelangelo immediately perks up. "Woah, is that a lightning bug?"

"In a sense, yes. It is a cockroach that I mutated with electricity using the white mutagen," she says. "The mutagen seems to only work if mutating an organic being with something inanimate, like electricity, and I believe the lioness mutant that attacked Raphael was mutated with the darkness surrounding her at the time of contact. A shadow mutant, if you will."

Casey stiffens as the others share surprised looks. Sarina sets the flask down on its side, letting the cockroach escape. It zips away into the night, a tiny little spark racing across the floor before disappearing in a flash.

"That would explain its speed, possibly its extremely low temperature," Donatello adds. "Just think of how quickly it gets dark when you shut off a light and how night is colder than the daytime, in most cases."

"Wait, then what about the exhaustion part? I felt like my energy drained the longer I fought with it," Destiny says.

"That...I do not know," Sarina says, "although it could just be a mutant ability. Severe drops in body temperature can cause sluggishness and fatigue, among other side effects, but that's just a theory at this point."

"So we've got a freaky super mutant on our hands?!" Michelangelo exclaims. He gasps, hopping in place. "Oh, oh! I know just what to call it!" He pauses, sweeping his hand over the air above his head. "Death Lion! No, no, wait! The Dark Lion Rises!"

"Lioness, technically," Donatello points out.

"Oh! I've got it. Ready? Ready for this!" He throws his hands out, fingers spread wide. "Beast Mode. Oh! There it is!"

"Thank you for being so quick with that, Michelangelo," Sarina says. "But, now comes the...rougher part. Donatello has let me know that Raphael is not improving. It seems he's gotten worse."

Casey's heart drops to his feet. The others stare in mute shock.

"Worse?" Destiny repeats, her voice tiny.

"How much worse?" Leonardo demands.

"Whatever Beast Mode did to him, it's like she gave him frostbite that just keeps getting worse on top of sapping all his energy," Donatello explains.

"But you can fix it, right?" Leonardo asks. Donatello and Sarina share a glance and he repeats, harsher, "Right?"

"The wound is already getting infected despite our work," Sarina admits. Casey notices her eyebrow twitch as she clasps her hands together so tightly that her knuckles turn white. "We can try to keep it clean but...as of now, the wounds aren't responding naturally."

"I could do the healing hands again," Leonardo suggests. "Or we could get Splinter to try? Maybe I did it wrong!"

"It didn't change anything, Leo. Not that we can see," Donatello says. "You might have slowed the infection, but at this point—"

"Could make an antidote?" Destiny interrupts.

"I suppose, given time, but—" Sarina sucks in a breath, starting to fidget. "But unfortunately, I have never come across a mutant like Beast Mode, and I...do not know where to begin."

Her voice cracks and she seems to deflate, dropping her face into her hands, and Donatello is quick to wrap her in a tight hug. She lets out a soft sob as she apologizes into his chest, over and over again, lamenting her failure. The others look at each other, disheartened.

Casey thinks he might be sick. If Sarina, the living computer and holder of more knowledge than he could ever imagine, doesn't know what to do, then what can they do? She said it herself, Raphael is getting worse. If they don't do something, if he doesn't do something, then he's going to lose his best friend.

He said he didn't want to see her again, but he's going to go back on that request. He has to.

"Maybe we can call Diaval?" Destiny suggests. "He could use his magic?"

"He doesn't have healing abilities, Des," Leonardo says. "Besides, what could he do from Canada? Raph would be dead before he could get on a plane!"

"We can't just do nothing! What if...what if I go back to that warehouse and we try to trap that mutant? Maybe—"

"And risk someone else getting hurt? Not a chance."

"I think I know someone," Casey says.

Immediately, all eyes fall on him and his skin crawls. Would they even believe him? He can't even guarantee that she can help.

"Who?" Michelangelo asks.

"It's a long shot. I don't even know if I'll be able to find her, but...right now, I think she's Raph's only chance."

She's a shadow mutant. Maybe she knows about the lioness. Maybe she knows what can soothe his injuries for long enough to figure out another solution. There are a whole lot of maybes but she's all he has to go on.

"Des, I'm gonna need my hoodie back," he says.

She nods and starts taking it off. Leonardo looks between them, mouth hanging open.

"Wait, Casey, who are you gonna find—" he starts.

"Just trust me, Leo." He takes the hoodie from Destiny and yanks it over his head as he starts for the turnstiles. "She's our only chance!"

"Casey!" Leonardo calls after him. "Who is she? How do you know her? CASEY!"

"Can't talk, Leo!" he shouts. "I've got a best friend to save."

----------

It's a long shot, a complete Hail Mary, and he looks insane, but it's all he has. It's so early that the sun hasn't started to rise yet, the night still blanketing the city, and he skates down the street searching every alley while he calls for her.

"Shadow? Shadow, it's me! Casey!" he calls.

Every alley, every shadowy patch, every dark space where your mind tries to fill in horrifying blanks the longer you stare at it, he searches for her.

He hasn't felt like this in months, so damn determined that he feels like he can't stop. He won't let himself attend another funeral. He won't let Raphael slip away when there's even a sliver of a chance that he could come back. He'd sooner die than let Raphael go.

But again and again, his cries are met with silence. He finds himself at the old hockey arena again, slowing to a stop near the bulletin board he tried to destroy only a short while ago. One of the missing dog posters has been stamped with the word "FOUND" and Casey can't help but wonder why they wouldn't just remove it, but hey, at least someone found who they were looking for.

He stares at the posters, lingering on one with two images of a girl. The one on the left is a photograph of a preteen, a brown-skinned girl with long coily black curls and a smile full of crooked teeth. The other is a rough sketch of what Casey imagines must be the first girl but aged up.

The poster is so old that he can barely read the writing anymore, just the big black headline declaring "MISSING."

"Yeah, I'm missing someone too," he comments.

He turns, looking out at the empty streets, the silent arena. If he made a poster asking her to meet him somewhere, what are the chances that she'd see it? She was here before. She must live in this area, right?

He cups his hands around his mouth. "Shadow! Please, if you can hear me, I need your help!" he pleads. "SHADOW!"

"Heya Hockey Puck, you rang?"

He's never been so happy to hear someone's voice. Shadow hovers before him, eyes bright, arms crossed over her chest, and he's suddenly overcome with the desperate need to hug her, so relieved that she's here. Instead, he grins.

"Shadow! Oh, man, am I glad to see you!" he says. "I've been looking for you everywhere. I really need your help."

She tilts her head a little. "What happened to your bandana?"

"My..." He touches his hair, suddenly remembering the absence of his favourite headgear. "Oh, yeah, I...uh...I had to use it for something else."

"You look different without it, that's all," she says. "What do you need?"

"I need you to come with me. My friend got attacked by a shadow mutant and—"

Her eyes seem to brighten for a second as they narrow. "Another shadow mutant?" she repeats.

"Yeah, a lion! She—"

"That isn't possible. Tell me you're lying."

"Why would I lie? We got attacked in this big warehouse, there was this white mutagen—" He shakes his head, recentering himself. "Look, that's not important. My point is, Raph is hurt real bad and we don't know how to make it better. I figured, you're a shadow mutant, maybe you'd know how to help! Maybe...uh..."

He trails off as she stares at him, eyes still narrowed and arms still crossed tightly over her chest. After a second, she turns her head away, her hair seeming to float after her in a delayed wave.

"I don't see how this is my problem," she says. "Just because I'm a shadow mutant doesn't mean I know anything about this. I don't get hurt anymore."

He gawks at her. "What? You didn't have a problem helping me with Brayden!"

"And?"

"And?! The hell do you mean and?! My best friend is dying!"

"And I'm not a doctor, puckhead," she says. "Your friend sounds doomed."

"Shadow, I swear I'm gonna—"

"What? Do what, Casey?" she demands. She floats closer to him, craning her head back as if to give him access to her throat. "You gonna grab me by the collar and shake some sense into my head? Drag me back to your friend until I'm forced to bend to your will? I dare you to try," she hisses. Their eyes meet, cold tickling his nose as she glares at him. "Go on, try. Humour me."

He curls his hands into fists, trembling. "I'm not gonna do that. I just...I thought you'd—"

"I can't help," she says. "You wasted your time."

She starts to fade and he lunges, swiping his hand forward, but she disappears before his eyes, leaving him with nothing but the goosebumps riddling his skin. He stares, hand outstretched, breaths getting faster as his vision starts to blur.

"FUCK!"

He turns and punches the bulletin board, crumpling the missing poster. He sinks to his knees, trying to brace himself on something, anything, as tears roll down his face. He tries to stop. He tries to hold his breath and squeeze his eyes shut and just stop crying, but it's a foolish errand.

"Please," he cries softly into the concrete. "Please, I don't know what else to do."

What are the chances that she's still around, lingering and watching?

"Shadow?"

Nothing. He lets out a choked sob even as he drags his arm across his eyes, rattling breaths through trembling lips.

"I'm sorry," he whispers. "I'm sorry, I don't know why I even tried. I...I just can't lose him." He's lost his dad, and April and Andy are gone. If he loses Raphael, he won't be able to take it. He'll completely lose himself. "He's my best friend. I'm just...I'm looking for an answer, or maybe a miracle? I just..."

A cool touch crawls across his shoulder and he inhales so sharply that it almost chokes him. He looks up and Shadow is there, hovering behind him with her eyes drooped. She looks...sad? Why would she be sad? He stares up at her in mute shock as she yanks her hand away from him.

"That was a test," she murmurs. "I'm sorry, but I don't exactly trust everyone's intentions. I've made that mistake before."

"A...a test?" he croaks. He coughs even as he struggles back to his feet. "Does that mean...?"

"I'll see what I can do for your friend," she says, "but you'll owe me."

He nods rapidly. "Anything!"

For a moment, her expression pinches as she stares at him. Then, after a second, she nods. "I'll let you know when I think of something," she mutters. "Lead the way."

"Okay. Awesome. Thank you, Shadow. Thank you so much."

She shakes her head even as they start down the sidewalk. "Don't thank me yet."