The search for the control room.
With enemy ships still in pursuit, Keyes summons SPARTAN-117, the Master Chief, from cryostasis and sends him to the surface of this new world in an effort to protect Cortana. But when Keyes attempts to land the Autumn on the ring's surface, the Covenant capture him. Disregarding military protocol, the Chief and Cortana infiltrate the enemy cruiser and free the captain, regrouping with the other UNSC survivors.
During the process they learn that the Halo installation is far more important than they could have ever imagined. The Covenant believed Halo was created by an ancient race of beings known as the Forerunners and that it was a weapon with immense power—something potentially capable of altering the dire fate the war promised.
Immediately, the Chief and Cortana set out for the installation's control room, but by the time they located it, another secret is revealed. The Covenant have unleashed a horror upon the ring known only as the Flood, a long-isolated parasite that feeds on any sentient life it can find, taking over their bodies and using them to infect others.