The level starts off with a cutscene of Master Chief observing the UNSC Infinity lose control and crash in the jungle. He proceeds to fight his way through Promethean forces to reach a Forerunner structure, where he reunites with Lasky and the Spartan-IV Sarah Palmer. Along with a few marines, the Chief clears out a landing zone for a Pelican to extract them.
He discovered the Covenant Remnant have allied themselves with the Prometheans. The Chief and a detachment of Spartan-IVs were landed near the Infinity, where they used an Scorpion and an Warthog to fight through Covenant forces, reaching the Infinity. The Master Chief then utilises a Mantis to clear out Covenant and Promethean soldiers to reach the weapon controls for the Infinity.
After clearing off several waves of Covenant Phantom dropships, he activates the weapons for the Infinity, firing at the Didact’s Ship and causing him to retreat. (Source)