From Halopedia, the Halo wiki
By the time that Crimson reaches their next objective, the marines of Boa Squad have already been wiped out by a Covenant assault. Crimson clears out the Covenant, while Roland attempts to trace Halsey's transmissions. Roland eventually locates the terminal that Halsey's transmissions repeatedly bounced off of. While Roland investigates further, Crimson comes under attack by massive Promethean forces. They hold off the machines long enough for Roland to complete his work. Miller orders Crimson to a safe location, then orders an airstrike which wipes out the Prometheans. Roland informs Crimson that Halsey's transmission was sent to the location known as "Lockup."
All dialogue is spoken through the COM.
Fireteam Crimson enters the battle. The two Marines from the intro have already been killed.
Once most of the initial Covenant are cleared out:
After all the Covenant are neutralized:
Crimson activates the designated terminal:
Crimson activates the second terminal:
Crimson reaches the third terminal:
Crimson activates the third terminal. At the same time, Covenant Phantoms and drop pods deploy enemy reinforcements. Crimson engages the attackers.
Once the Covenant troops are defeated, Prometheans begin phasing onto the field.
More and more Prometheans phase onto the battlefield.
Crimson continues to hold out against the Promethean onslaught.
When only a few Prometheans are left:
Once the last Promethean is down:
A new wave of Promethean Knights rapidly teleports in.
Crimson heads to the designated area Roland has indicated. Behind them, a massive aerial bombardment commences, wiping out the Prometheans.
Fade to black.