From Halopedia, the Halo wiki
After the UNSC Spirit of Fire is damaged by a collision with a CPV-class heavy destroyer, Sergeant John Forge is charged with repairing the ship's power core while under heavy attack. This is the second and final level where players can control Cyclopes, mainly to repair the ship's power core and the third and final level where the player can control Gremlins.
The UNSC Spirit of Fire emerges inside the inner Dyson sphere of the Etran Harborage shield world and almost immediately ends up on a collision course with the Covenant CPV-class heavy destroyer Proclamation's Tithe. Despite the ship quickly adjusting course, the port side of the Spirit of Fire collides with the destroyer, heavily damaging the ship and destroying several Herons. The Proclamation's Tithe opens fire on the Spirit of Fire as Captain James Cutter orders repair teams led by Sergeant John Forge to the top deck and Serina notices that they are actually inside the planet.
Commanded by Forge, several Cyclopes set to work repairing the ship's damaged power core with the damage preventing the Spirit of Fire from escaping. As the repairs are conducted, the destroyer sends over multiple boarding parties using Spirit dropships as well as Banshees and Locusts. Troops and vehicles from the Spirit of Fire struggle to repel the heavy assault while repairs are made and the destroyer continues to exchange fire with the Spirit of Fire.
Finally, after the ship's power core is repaired, Captain Cutter orders Serina to take the Spirit of Fire away at best speed and the ship escapes the Covenant destroyer.
Scene opens to a calm, pine-forested area surrounded by Forerunner structures clustered around an opening in the ground, presumably designed for entry into the interior of the shield world. Soon, the ground begins to quake violently, before the massive frame of the UNSC Spirit of Fire emerges from said entrance port at high speed. The UNSC vessel is then revealed to be heading straight towards the Covenant CPV-class heavy destroyer Proclamation's Tithe.
Inside the bridge of the Spirit of Fire, Captain Cutter is seen sitting in his command chair while looking through the central window of the bridge, quickly examining the approaching Covenant destroyer, and turning around to Serina.
Despite course corrections, the Spirit of Fire fails to completely avoid the incoming ship, and the destroyer violently shears through the port side of the Spirit of Fire, destroying its docked supply of Herons. The destroyer itself remains completely unscathed from the violent collision, protected by its energy shields, and begins to slow down along with the Spirit of Fire, putting the two vessels hovering directly next to one another. On the bridge of the Spirit of Fire, the combat alarm begins to sound.
The destroyer fires its plasma turret from the side of its hull at the Spirit of Fire sending shards of wrecked metal into the air, while several deck-mounted cannons begin to emerge from within the Spirit of Fire. Back on the bridge in the Spirit of Fire, Serina is looking down at a the holo-table, which shows a holographic display of the two ships, which are then revealed to be on the interior of a hollow construct.
The newly emerged deck-mounted cannons begin to fire upon the destroyer, which responds with more heavy plasma fire, destroying one of the deck guns and filling the screen with explosions.
On the port side of the Spirit of Fire, UNSC forces regroup, while Banshees and Hawks fight each other over the ship.
Sergeant John Forge and his Marines are seen accompanying a Cyclops to the power core.
Cyclopes begin to repair the power core, while Covenant dropships land and deploy troops. The UNSC forces fight back the invading forces. Covenant Banshees start to fire on the power core.
The battle rages on as the Covenant destroyer begins to support the troops with its plasma turrets. When the power core has been repaired halfway:
If the Covenant have significantly damaged the power core while it's under repair:
The firefight on the hull continues. The power core is nearly repaired.
The UNSC forces continue to fight off the Covenant until the power core has been full repaired.