Inside a Forerunner dome, a Grunt walks down a circular path as all his comrades were sleeping. Suddenly he stops and hears something before jumping and screaming in fear as the Pelican rises up.

Foehammer: (COM) This is as far as I can go.

Cortana: (COM) Roger that, we'll be able to find our way to the Control Center from here.

Foehammer: (COM) Good luck. Foehammer out.

The Pelican hovers at the edge of the platform, and Chief and Y/n gets out. They then start going through a series of circular rooms, clearing out the Covenant.

Y/n: Man, why do so many of these rooms look the freakin' same?! That's another thing to add to the list of Forerunner mysteries: why quantity instead of quality?

Cortana: The Covenant presence here is stronger than I anticipated. They seem to have the entire region secured.

They emerge onto a bridge, where they then see it's snowing.

Cortana: Interesting... the weather patterns here seem natural, not artificial. I wonder if the ring's environment systems are malfunctioning... or if the designers wanted the installation to have inclement weather.

Y/n: I'm seriously resisting the urge to make a snow angel.

A Pelican flies over the bridge as Covenant forces open fire.

Fireteam Zulu Sergeant: (COM) This is Fire Team Zulu requesting immediate assistance from any UNSC forces. Does anyone copy? Over.

Cortana: I didn't think there were any human forces left on this part of the ring. (COM) Cortana to Fireteam Zulu, I read you. Fire Team Zulu, this is Cortana. Hold position. We're on our way.

Fireteam Zulu Sergeant: (COM) Roger that. Make it quick.

The Pelican descends to an area below the bridge.

Chief and Y/n enter another structure and descend to the ground level, where they return to the canyon. A large number of Covenant have three Marines with an overturned Warthog cornered. A Covenant Wraith tank fires plasma mortars as Shade turrets and Ghost vehicles strafe the Marines.

Y/n: Cavalry's here, boys!

Marine #1: Look! It's the Chief and the Lieutenant!

Marine #2: Glad you could join the party, sirs!

Chief hijacks the Wraith and rips the canopy hatch off before punching the driver and dropping a grenade in. He leaps off as the Wraith explodes and lands on a Ghost, kicking the Elite out and mowing down the remaining ground forces.

Y/n spots a spare rocket launcher and loads it, blowing up the Shade turrets that had pinned Fireteam Zulu.

Marine #3: They gave it their best shot, but they didn't count on running into the biggest squad of Marine bad-asses in the Corps!

Fireteam Zulu Sergeant: That's the last of 'em. Area secure. Stand down, team! They did their damnedest, that's for sure. This is all that's left of my platoon! The Covenant are down here in force... and they brought plenty of toys! I wouldn't have given us a snowball's chance until you showed up, sirs.

Y/n: We're trying to get to Halo's control room. We could use some backup.

Marine #2: What?! We need to get the hell outta here, man! Somebody call the dropship!

Fireteam Zulu Sergeant: You stow that garbage right now, mister. You're a Marine, so act like it, dammit! We've got a job to do.

Cortana: (COM) Negative, team. No evac available at this time. But the Chief, and the Lieutenant, and I could use some assistance.

Fireteam Zulu Sergeant: Okay, Marines. Time to pay back the Chief and the Lieutenant for pulling your sorry tails out of the fire. Prepare to move out!

The group reaches a second large open area, where one Wraith hovers on the ice and another lurks on top of the cliff at the far end of the canyon.

Marine #4: Bloody hell! Enemy forces have armor support... it's time to earn our pay.

Cortana: The Covenant placed their tanks to defend the entrance to the tunnel.

Chief takes Y/n's rocket launcher and blows up the first Wraith before using the Ghost he'd stolen to zoom up the cliff and take out the second one with the last rocket.

Y/n: Whoo! That's what I'm talkin' about!

Marine #1: Looks like that armor wasn't so tough after all! Let's mop up the rest of 'em!

They clean out the rest and make they're way into a tunnel, encountering more ground troops.

Marine #2: More Covenant sighted! Man, these guys are like cockroaches...

Y/n: Yeah, and we're Pest Control! Reload, boys!

The group crosses the cavern and exits into another tunnel, which soon turns into a rocky slope heading upwards. Another Wraith started attacking, but the Chief takes it down without a sweat.

Fireteam Zulu Sergeant: You monkeys almost look like golden boys!

Marine #4: Damn straight!

A pair of Hunters chase  after a squad of Marines and Sergeant Stacker, where the path switches back and goes down to a frozen river leading to the cave to the last area of the second chasm.

Cortana: I thought the Covenant had eliminated all the Marine forces in this area.

Chief engages and defeats the Hunters, as a Covenant Spirit appears from behind the cliffs and drops in on the ridge he just descended from.

Y/n: Anybody else hear that? Aw, man... Covenant dropship inbound, take cover!

Marine #3: The Covenant are dropping in behind us! We're trapped!

Chief and Y/n near some switchbacks leading up to final area of the second chasm.

Marine #2: Master Chief, Lieutenant L/n, the Covenant have stationary guns dug up in those rocks!

Cortana: We need to get up this ridge. Marines, hold your positions. We'll handle those guns. Master Chief, L/n, we should scale the ridge and get below those stationary guns.

Fireteam Zulu Sergeant: Roger that. Marines, stay here and cover the Master Chief and Lieutenant.

The duo eliminate all enemies in the area, including a Banshee and two Ghosts, and approach a door in the cliffs.

Cortana: (COM) Cortana to Fire Team Zulu. I've sent a distress signal tagged with your current position. Hold your position and await evac. The three of us are going to continue on ahead.

Fireteam Zulu Sergeant: That's affirmative! Fire Team Zulu out! Good luck, sirs!

Chief and Y/n battle their way through more interior areas and across two more bridges.

Cortana: Resistance appears to be increasing. We must be close to the Control Center.

Y/n: I really hope so, ma'am. I can't take much more of this cold.

They then exit a large door to a third chasm.

Cortana: Analyzing. This must be the control room. Subtle. The entrance to the control room is at the top of that pyramid structure. Bring me up there and I should be able to get us inside.

Y/n: Um, "should"?

Cortana: Have a little faith, trooper.

Y/n: (mutters) I have a little.

They then make their way across the chasm, taking down several Covenant squads.

Cortana: The entrance to the Control Room is at the top of the pyramid. Let's get up there. We should commandeer those Ghosts, we're going to need the firepower.

Y/n: Um, I don't know how to drive those, ma'am. I didn't take any Covenant vehicle simulations.

Chief: Then hold on tight, trooper.

Chief sits Y/n in front of him and works the Ghost's controls as they shoot up the pyramid.

Y/n: This just feels wrong, no offense, Chief!

They make it to the top and take out the remaining Covenant.

Cortana: Scanning... Covenant forces in the vicinity have been eliminated. Let's move on to Halo's control room.

The Chief and Y/n walk through the door and to a control panel at the center of a circular glass walkway projecting into the middle of a massive open chamber. A holographic map of Halo surrounds a smaller holographic map of the system.

Y/n: (whistle) If this doesn't qualify as a control room, then I don't know what does.

Cortana: That terminal, try there.

Chief unplugs Cortana's chip from his helmet and places her chip in the terminal. A moment later, she appears above the display. Data streams out from her in all directions. Her eyes are now red and her body is green, shifting occasionally to blue and purple.

Chief: You all right?

Cortana: (smiles) Never been better! You can't imagine the wealth of information. The knowledge... so much... so fast. It's glorious!

Y/n: So, if Halo's a weapon, what kind of weapon is it supposed to be?

Cortana: What are you talking about?

Chief: Let's stay focused. Halo... how do we use it against the Covenant?

Cortana: This ring isn't a cudgel, you barbarians, it's something else... something much more important.

Y/n: Barba-?! We just asked a simple question! Geez! You sure interfacing with that terminal hasn't made you go all screwy or something?

Cortana: No, it hasn't. The Covenant were right... this ring... it's Forerunner. Give me a second to access... yes, the Forerunners built this place, what they called a "fortress world," in order to... no, that can't be. Oh, those Covenant fools, they must have known, there must have been signs!

Chief: Slow down. You're losing us.

Cortana: The Covenant found something, buried in this ring, something horrible, and now... they're afraid.

Y/n: Buried? What could they have found that-

Cortana: The Captain, we've got to stop the Captain!

Chief: Keyes? What do we—

Cortana: The weapons cache he's looking for. It's not really— We can't let him get inside!

Y/n: Cortana, you're not making any sen-

Cortana: There's no time. Get out of here, find Keyes, and stop him! Before it's too late!

The Chief and Y/n look at one another before turning and running from the control room.

(MAN, CHIEF AND Y/N ARE IN FOR A DOOZY!)