From Halopedia, the Halo wiki
With Requiem headed towards the sun, Fireteam Crimson returns to the Covenant dig site to find one of the two artifacts binding the UNSC Infinity to the planet. Though most of the Covenant have evacuated Requiem, some true believers have stayed behind to continue the fight. Crimson wipes out the Covenant guarding the dig site, then searches for the artifact using Glassman's coordinates. When all they find is a rock wall, Doctor Glassman informs them that the artifact is hidden somewhere behind the wall. Miller then realizes the Covenant must have been after the artifact as well, explaining why they brought the Harvester that Crimson had previously disabled.
Miller orders Crimson to get onboard the Harvester and finish what the Covenant started. However, the Harvester's entrance is protected by an energy shield. Miller wants to call in a Pelican to bring down the shield, but Dalton informs him that there are enemy anti-aircraft guns nearby. Miller sends Crimson to destroy the guns. Though the Forerunner AA guns are shielded, Roland is able to hack into them and bring the shields down. Dalton sends Crimson some heavy weapons, which they use to destroy the Forerunner AA guns. Crimson then fights their way back to Harvester, defeating several waves of Covenant reinforcements. When they reach the Harvester, Murphy's Pelican arrives and shoots the Harvester's shield, bringing it down.
Crimson boards the Harvester and clears out the Covenant crew. They also rescue some captured scientists who the Covenant were trying to force to repair the Harvester. The scientists inform the Spartans that the Harvester's power supply is destroyed. Roland says he has an idea of where they can get a new power source.
Crimson engages the Covenant forces on the ground. Murphy’s Pelican provides some cover fire. Once the immediate area is clear, Crimson heads toward the waypoint.
After clearing out the heavy Covenant presence guarding the area, the Spartans move to the waypoint, but find nothing of interest.
The entrance to the Harvester is blocked by an energy shield.
New waypoint is set. Crimson heads off for the anti-air guns. They kill any Covenant in the way.
Upon reaching the Forerunner AA guns:
Drop pod descends and deploys more enemies. Crimson dispatches them.
Heavy weapons are dropped off for Crimson.
Crimson uses the heavy weapons to take out two AA turrets.
Crimson approaches the third AA gun.
An enemy Phantom rises over the ridge.
The Phantom deploys a pair of Hunters. Crimson defeats them both.
Crimson destroys the final AA gun.
Drop pods land in front of Crimson and deploy more enemy troops. Crimson dispatches them all and makes their way back to the Harvester. Sangheili and Kig-Yar snipers try to stop them, but fail.
Murphy’s Pelican arrives and blasts the Harvester’s shield, bringing it down.
Crimson moves into the Harvester and engages the Covenant inside.
Crimson clears out the Harvester.
Crimson deactivates one of the control consoles for the shield wall trapping the scientists.
Crimson deactivates the shield wall trapping the scientists.
The scientists evacuate, while Crimson investigates the Harvester's power supply. The damage inflicted in the previous raid is still present.
Crimson tries to activate a damaged control console. The console makes a groaning sound and spews out sparks. It's clearly not functioning.
Crimson exits the Harvester. Murphy’s Pelican is waiting for them outside.
Fade to black.