Master Chief, Y/n, and Guilty Spark teleport in the Control Room, with Spark in the middle of a conversation, explaining about the Flood.

Guilty Spark: ...which means that any organism of sufficient mass and cognitive capability is a potential vector.

Chief and Y/n are silent, both feeling as if something isn't quite right.

Guilty Spark: Is something wrong?

Chief: No... Nothing.

Y/n: Yeah, all good.

Guilty Spark: Splendid. Shall we?

Chief and Y/n walk after Guilty Spark as he moves up to the main control panel, humming.

Guilty Spark: Unfortunately, my usefulness to this particular endeavor has come to an end. Protocol does not allow units with my classification to... perform a task as important as the reunification of the Index with the Core. That final step is reserved for you, Reclaimers.

Chief takes the Index and looks at Y/n who shrugs as he then puts it into a slot on the console. Lights and holograms appear, but then fade.

Guilty Spark: Odd. That wasn't supposed to happen.

Cortana suddenly appears over the console.

Cortana: Oh, really!?

There is a flash of light, and Guilty Spark is thrown back, and down to the ground.

Chief: Cortana!

Y/n: Why did you-?!

Cortana: I've spent the last 12 hours cooped up in here, watching the two of you toady about, helping that thing get set to slit our throats!

Chief: Hold on now, he's a friend.

Cortana: (mockingly) Oh! I didn't realize. He's your pal, is he? Your chum? (angrily) Do you have any idea what that bastard almost made you do?!

Y/n: Uh, yeah? Activate Halo's defenses, and destroy the Flood. That's why we brought the Index to the Control Center.

Cortana: You mean this?

A hologram of the Index appears in Cortana's hand.

Guilty Spark: A construct?! In the Core?! That is absolutely unacceptable!

Cortana: Sod off!

Guilty Spark: What impertinence! I shall purge you at once!

Cortana: (smiles nastily) You sure that's a good idea?

Cortana's avatar absorbs the Index.

Guilty Spark: (outraged) "How... how... how dare you! I'll--"

Cortana: Do what? I have the Index! You can just float and sputter!

Y/n stands between them, hands raised.

Y/n: BOTH OF YOU! STOP IT!

Chief turns to Cortana.

Chief: Cortana, the Flood is spreading. If we activate Halo's defenses, we can wipe them out.

Cortana: You two have no idea how this ring works, do you? Why the Forerunners built it? Halo doesn't kill Flood, it kills their food. Humans, Covenant, whatever. We're all equally edible. The only way to stop the Flood is to starve them to death. And that's exactly what Halo is designed to do: Wipe the galaxy clean of all sentient life. You don't believe me? Ask him!

Cortana nods at Spark, making the two humans turn to the Monitor.

Chief: Is it true?

Guilty Spark: (pauses) More or less. Technically, this installation's pulse has a maximum effective radius of twenty-five thousand light years. But, once the others follow suit, this galaxy will be quite devoid of life, or at least any life with sufficient biomass to sustain the Flood.

Y/n: You lied to us! WHY?!

Guilty Spark: Because I assumed you both knew that... I mean, how couldn't you?

Cortana: Left out that little detail, did he?

Guilty Spark: We have followed outbreak containment procedure to the letter. You were both with me each step of the way, as we managed this crisis.

Cortana: (alarmed) I'm picking up movement!

Guilty Spark: Why would you hesitate to do what you have already done?

Cortana: (urgently) We need to go, right now.

Four Sentinels float up behind Guilty Spark, their beams aimed menacingly at Chief and Y/n.

Guilty Spark: Last time, you asked me, if it were my choice, would I do it? Having had considerable time to ponder your query, my answer has not changed.

Chief and Y/n look around at the Sentinels, and Chief backs towards the panel. He takes Cortana's disk out of the console, and inserts it into his helmet as Y/n backs up to his side, not taking his eyes off the machines.

Guilty Spark: There is no choice. We must activate the ring.

Cortana: (in Chief's head) Get - us - out - of - here!

Guilty Spark: If you are unwilling to help, I will simply find another. Still, I must have the Index. Give your construct to me, or I will be forced to take her from you.

Chief: That's not going to happen.

Guilty Spark: So be it. (to the Sentinels) Save his head. Dispose of their bodies.

Guilty Spark teleports away, and his Sentinels attack.

Cortana: Look out!

Chief and Y/n both dive out of the way ad the beams shot by the Sentinels, before shooting them down with their weapons.

Cortana: We can't let the Monitor activate Halo. We have to stop him - we have to destroy Halo. According to my analysis of the available data, I believe the best course of action is somewhat... risky. An explosion of sufficient size will help destabilize the ring, and will cut through a number of primary systems. We need to trigger a detonation on a large scale, however. A star ship's fusion reactors going critical would do the job. I'm going to search what's left of the Covenant battlenet, and see if I can locate the Pillar of Autumn's crash site. If the ship's fusion reactors are still relatively intact, we can use them to destroy Halo.

Y/n: Man, I could go for a nap after all this is said and done.

They make their way outside, after taking down several squads of Covenant.

Cortana: I haven't located the crash site yet but we need to buy some time, in case the Monitor or his Sentinels find a way to activate Halo's final weapon, without the Index. The machinery in these canyons are Halo's primary firing mechanisms. They consist of three phase pulse generators that amplify Halo's signal, and allow it to fire deep into space. The power levels are enormous. I can't even begin to calculate the pulse's range. So, if we damage or destroy these generators, the Monitor will need to repair them before Halo can be used. That should buy us some time. I'm marking the location of the nearest pulse generator with a nav point. We need to move in and neutralize the device.

Y/n: Hey, down there!

Chief follows Y/n's gaze and sees several unused Banshees.

Cortana: Wait. We should commandeer those Banshees; we'll need it to reach the pulse generator in time.

Y/n: Again, I have no experience in piloting Covenant vehicles, ma'am.

Cortana: The only other option is you hang on to the wing of the Chief's Banshee, Lieutenant.

Y/n: ....I hope to God there's some kind of flight assistance on those things.

Chief and Y/n slide down the slopes of Control Room's sides and make it to the Banshees.

Cortana: Good, let's get moving. The nav point marks our target.

Chief and Y/n enter their vehicles and ascend upward. Y/n's Banshee shakes as he whimpers and grips the controls tightly.

Y/n: Got it, got it. WHOAH!

Y/n's Banshee does a flip, making him hold the contents of his stomach down.

Y/n: (swallows) Why the hell do they have the controls on inverted?!

They then make it to the first generator room.

Cortana: That's the pulse generator. The center core is the signal amplifier. That's what we need to shut down. We need to interrupt the pulse generator's energy stream. I've adjusted your shield system so that it will deliver an EMP  burst to disrupt the generator. You will need to walk into the beam, to trigger it. The EMP blast should neutralize the generator. But it will also drain your shields, and you will be vulnerable until they recharge.

Chief enters the generator and disables it.

Cortana: Objective accomplished! The pulse generator has overloaded. Scanning... the generator's central core is offline. Well done.

Y/n: Did that feel weird at all?

Chief: Kind of tingled.

Y/n: Huh.

Cortana: The second pulse generator is located in the adjacent canyon. Move out, and I'll mark the target with a nav point when we get closer.

More Flood and Covenant attack, but the pair get through them and take a bridge outside.

Cortana: Head through that door. That's the way to the next canyon.

They find two more Banshees and take them up towards the next generator.

Y/n: Hey, I think I'm getting the hang of- THIIIIIISSSSS!

Y/n shoots forward, having activated the Banshee's thrusters and narrowly avoids clipping his right wing against the canyon's wall.

Y/n: (exhales in relief) Jesus!

They then find the entrance to the next generator in the canyon wall and land. They take care of the Flood guarding it and Chief deactivates it.

Cortana: That did it! The pulse generator overloaded. One more to go.

Once they're back outside, they jump back in their Banshees.

Cortana: I've located the Pillar of Autumn. She put down twelve-hundred kilometers up spin. Energy readings show her fusion reactors are still powered up. The systems on the Pillar of Autumn have fail safes that even I can't override, without authorization from the Captain. We'll need to find him or his neural implants, to start the fusion core detonation. One target remaining. Let's take care of the final pulse generator.

Y/n: I hope the Captain's alive. We never found his body at that structure or fought a Flood with his uniform, so hopefully there's a chance.

They then spot a tunnel down below.

Cortana: That tunnel leads to the next section of the canyon. The final pulse generator we need to destroy is through there.

The Spartan and ODST make it into the third canyon and are attacked and armada of both Flood and Covenant with supporting vehicles. They find the final generator and Chief takes it offline.

Cortana: Final target neutralized. Let's get out of here.

Chief: Let's find a ride, and get to the Captain.

Cortana: No, that'll take too long.

Y/n: How else are we gonna get there then?

Cortana: There's a teleportation grid that runs throughout Halo. That's how the Monitor moves about so quickly. I learned how to tap into the grid when I was in the Control Center. Unfortunately, each jump requires a rather... consequential expenditure of energy.

Chief: Something tells me I'm not gonna like this.

Y/n: Seconded.

Cortana: But I'm pretty sure I can take the needed energy from your suit without permanently damaging your shields. Needless to say, I think we should only try this once.

Chief: Do it.

Y/n: (pleading) Are you sure we can't just hijack a Spirit or something?

Y/n turns around and holsters his SMG as Chief holds his shoulders and they teleport.

(MAN, KEYES' TERMINAL IN THE NEXT LEVEL ALWAYS FREAKED ME OUT!)