Master Chief and Cortana took one of the Broadsword with nuke ready as it took off in high speed.

Cortana: Approaching the Didact's ship in two hundred kilometers! Once we get onboard, we'll find the bridge.

A slipspace portal opens in front of the Mantle's Approach.

Master Chief: He's on the move again.
Cortana: The fighter's shields aren't rated for slipspace!
Master Chief: No. But the Didact's are.

John pushes the throttle. The fighter boosts forward and approaches the Didact's ship. As it nestles on the top, the shields of the Mantle's Approach activate, and the enormous ship and Broadsword pass into slipspace together.

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Master Chief: That's the last one. Infinity, you're clear.

the Infinity, positioning itself to face the Didact's ship.

Lasky (COM): Roger that, Chief. You might want to back up a little. Main battery, fire!

The Infinity fires its twin fore weapons, which strike the front of the Mantle's Approach and create a neat hole leading to the interior. The Chief's Broadsword flies back to enter the ship.

Master Chief: Clean hit. We're proceeding to insertion.
Lasky (COM): Acknowledged. We'll be on station if you need us. Make sure you give the Didact our regards. Infinity out.

The fighter curves around and dives inside the still-burning hole. Inside it's dark and a tight fit. A wall tries to close to block the Chief's way

Cortana: Chief, look out!

The Broadsword spins out of the wall's way. More walls start closing in to stop their fighter. Cut to the Broadsword's cockpit. John can see the tunnel growing tighter and tighter.

Cortana: I don't think this is going to end well!

as the fighter crashed. John jumps out.

Cortana: Now what do we do?
John turns around and carefully removes the warhead from the missile. He then locks onto the back of his armor and grabs his assault rifle.

Master Chief: Plan B.

He continued to walk and fight his way to Didact's Location until Finally made it to the Composer.

Cortana: Chief, once that warhead is primed, the window for getting out of here is going to be pretty slim.
Master Chief: I know.

The room's roof opens up, revealing Earth in the sky above. An enormous slipspace portal forms in the chasm below the machine.

Didact: And so, you come at last.
Cortana: Activity! Significant slipspace event, building under the Composer.
Master Chief: He's powering it up.

John lands on a platform with a hallway leading further in. After he passes through the door, he sees an orange energy shield now surrounding the Didact and his machine. The shield is projected by two beams at opposite ends of the room.

Cortana: The Didact's shielded himself inside the Composer. The nuke won't do us any good unless we can disable that barrier. Find me a terminal.

John plugs her into a terminal at the end of the platform. Cortana's image appears, now flickering badly.

Cortana: I've got to do something you're not going to like.
Cortana screams and her body contorts in pain. Each time she steps to the side, another Cortana comes out of her and disappears into the terminal. John then retrieves her.

Master Chief: What did you just do?
Cortana: I ejected my rampant personality spikes into the system. If I do that at each of those beams, the copies can overwhelm the Composer's shielding. Get ready!

He fought the Promethean Knights and made it to the terminal at the same time, Cortana did the same as she did. After that later, the shields are now disabled.

Didact: And yet, still you fail.

With a wave of his hand, the Composer activates and fires at Earth, seen in the slipspace portal above. The beam strikes in the North American southwest.

Didact rises into the beam itself, while enormous energy from it is seen surging below the machine.

Cortana turns to face John. John tries to retrieve her but the terminal suddenly disintegrates, knocking John away and draining his shields. Much in horror to Master Chief.

Master Chief: Cortana!

Then he checked on the HUD. It is Cortana's Message.

Cortana: It's alright. But you must hurry...

He proceeds to the composer, cautiously walks down the bridge, keeping an eye out for the Didact. Looking into the beam, he sees the Didact inside vanish. He continues to walk forward, then turns and aims his weapon when he hears the Didact speak.

Didact: You persist too long after your own defeat.

Master Chief levels his weapon, but can't see the Didact anywhere. Seemingly alone, he cautiously backs away, then continues down the bridge. Behind him, the Didact floats down.

Didact: Come then, Warrior. Have your resolution.

John turns to fire on him, but the Didact telekinetically bats him away. John's battle rifle falls off the bridge and the nuke falls toward the end of it.

John skids to a stop, turns to look at the Didact, then at the nuke. He charges toward the latter, but the Didact again telekinetically stops his lunge. The Forerunner lifts him midair and floats John towards him, removing his own helmet as he does so.

Didact: So misguided.

The Didact floats John off the edge of the bridge. John is unable to do anything to free himself. His shield flickers from the constraint.

Didact: Humanity's imprisonment is a kindness.

He tightens his fist, and John begins to choke. Hearing a strange whispering, the Didact looks and sees multiple Cortanas rising from his light bridge.

Cortana(s): In that case, you won't mind if we return the favor.
Didact: Your compassion for mankind is misplaced.
Cortana(s): I'm not doing this for mankind.

The seven Cortanas leap into his armor and form chains of hard light that hold down his arms and legs. The Didact loses his grip on John, who falls and manages to catch the edge of the light bridge. He looks up. John struggles to climb up the edge. His shields are down and don't recharge. After looking down at the Composer's energy below, he pulls himself up and pulls out a pulse grenade. The Didact is still struggling to free himself. He dashes toward the Didact and plants the grenade in his chest. The Didact breaks free and punches him away at that moment. The Didact again telekinetically strangles John, and the Spartan is about to lose his breath when the pulse grenade detonates. The Didact reels from the blast lost his balance, and falls off the bridge, dropping into the swirling energy below. Master Chief watches the Didact vanish into the vortex, then groans as he crawls to the HAVOK bomb. He tires and eventually can only pull himself with his arms. Grabbing the nuke and priming it, he looks up at the beam firing on Earth and decides there's no time to escape. With a loud cry, John punches the bomb to make it immediately explode.

John comes to inside a purple bubble, which seems to be made out of hard light. He looks around him then stands on his feet.

Master Chief: Cortana? Cortana, do you read? Cortana, come in.

Johns turns around and sees Cortana striding towards him. Her body is made out of hard light, and she's now as tall as a real human being. The two of them walk towards each other.

Master Chief: How...?
Cortana: Oh, I'm the strangest thing you've seen all day?
Master Chief: But if we're here-
Cortana: It worked. You did it. Just like you always do.

Cortana smiles. John looks around the bubble curiously.

Master Chief: So how do we get out of here?

Cortana looks down at the floor a little gravely.

Cortana: I'm not coming with you this time.

Master Chief couldn't believe in his eyes that Cortana is not coming with him.

Master Chief: ...what?
Cortana: Most of me is down there. I only held enough back to get you off the ship.

He was still in disbelief.

Master Chief: No. That's not-! We go together.
Cortana: It's already done.

Master Chief is having a bit emotional.

Master Chief: I am not leaving you here.
Cortana: John...

Cortana walks up to him and touches his breastplate. The spot glows in response.

Cortana: (sighs) I've waited so long to do that.

John looks away.

Master Chief: It was my job to take care of you.
Cortana: We were supposed to take care of each other. And we did.

John looks up. Cortana sadly smiles.

Master Chief: Cortana- please...

Cortana begins to back away, out of the bubble.

Master Chief: Wait.
Cortana: Welcome home, John.

Cortana steps out of the bubble and vanishes. John, staring at where she was as the Didact's ship crumbles around him. Cortana is now gone and she was never coming back.