From Halopedia, the Halo wiki
Fireteam Crimson is held captive by the Covenant at "Lockup" following their capture. Miller and Dalton locate them and distract the Covenant, allowing the team to break free. However, Dalton is unable to extract Crimson since they are too deep behind enemy lines. Crimson instead looks for a ship to hijack. During the search, they discover a squad of UNSC Marines being held captive. They free the marines, and the squad leader, Lieutenant TJ Murphy, offers to help hijack a Covenant ship. Crimson and the marines provide cover fire for Murphy, while he remotely seizes control of a Phantom. The humans then use the captured Phantom to escape.
Crimson receives an incoming transmission:
An explosion in the background causes the guards to turn their backs. At the same time, the energy chains lose power and Fireteam Crimson is released. They attack the guards from behind, kill them, and grab their weapons.
Crimson engage the other Covenant guards.
A Covenant Phantom arrives and drops off enemy reinforcements.
Once most hostiles have been eliminated:
After all hostiles are down:
Crimson takes out the shield controls.
More Covenant troops appear.
Crimson heads to the evac zone. A Covenant CRS-class light cruiser holds position directly above.
More Covenant arrive.
After the area is almost clear:
Once the area is secure.
Crimson heads through a door and fight their way to the objective. However, the Phantom they intend to hijack takes off before they can reach it.
Crimson reaches a marked door.
Crimson enters through the door. Sangheili and Promethean Crawlers are waiting for them on the other side.
Crimson engages the enemy. After clearing out the chamber, Crimson discovers a small group of marines trapped behind a shield wall.
Crimson deactivates the shield wall, freeing the marines trapped inside.
Crimson and the marines defend Murphy from the Covenant attackers while he works on the override controls. An incoming Phantom opens fire, but suddenly ceases its attack as Murphy begins remotely hacking it. Rampant energy bolts surge across its hull and it drifts aimlessly. Murphy soon has the Phantom under his control.
Crimson finishes off the last Covenant.
Murphy and Crimson board the hijacked Phantom.