From Halopedia, the Halo wiki
Fireteam Raven is redirected from the Battle of Alpha Base by Wellsley to assist the Master Chief on his secret mission. During the mission, the fireteam punches a hole in Flood forces, allowing the Spartan to proceed with his own objectives.
The Falcon nears a bridge in the canyon, before the camera cuts to gameplay, with players manning the M247H machine guns. The craft is attacked by two waves of four Type-26 Banshees, before flying over the aforementioned bridge. Multiple Flood combat forms can be seen on the bridge. Many more Banshees attack as the Falcon begins to descend. The Falcon then ascends again to hover over the bridge, infested with Flood forms.
The Falcon sweeps the bridge, with the players clearing off a myriad of Flood. A Banshee being hijacked by Flood forms crashes into the bridge. The Falcon fights more Flood by once again being enagged by Banshees. On the canyon floor, dozens of Flood can be seen running towards the Falcon, with the pilot giving the players a clear shot. The craft then turns around to reveal a small encampment of marines attempting to fight off the assailants. The four marines have two turret-less M12 Warthogs.
Fireteam Raven exit the Falcon, now equipped with M6G magnums. They fight off several combat forms, before two Flood tank forms smash through the Warthogs and kill the marines. Two more follow before a Dextro Xur-pattern Spirit looms overhead, supported by Banshee airstrikes. The final remaining marine and the fireteam are assailed by combat forms and more tank forms, before picking up M41 SPNKRs. The rocket launchers are used to fight off several Banshees who are using their Fuel rod cannons to conduct airstrikes. Several Type-32 Ghosts enter the battle, supported by a Type-26 Wraith. Following the Wraith's destruction, the ODSTs switch back to their MA5B assault rifles as the Falcon flies overhead. The marine is stood next to the flipped-over Warthog.
Raven use their rifles to mow down waves of combat forms, until a tank form jumps onto a rock in front of them. As the ODSTs engage, the camera goes into slow motion as John-117 jumps up from behind, firing his M90 shotgun into the creature's back. Exiting slow-mo, the Spartan lands on the creature's back and fights off more Flood forms. He then nods to the fireteam, before running off to continue with his mission. The Falcon flies past again, revealing several Flood carrier forms and Pod infectors assailing the group. The marine is killed by an infection form, as the rest are killed. The Falcon lands in the distance, and the group begin making their way to it. Dozens more infection and combat forms are killed before the group picks up a shotgun off a corpse. More Flood of various forms are encountered before the fireteam finally gets to the chopper, to find several combat forms atacking it. They are cleared off, but some Covenant infantry comrpised of Sangheili and Unggoy engage from behind. A Wraith then engages, with Raven boarding the Falcon to escape.
The Wraith's Plasma mortar fire hits the Falcon, causing the craft to begin smoking. The mission then ends with the Falcon exiting the canyon system.