In 2552, humanity is losing a genocidal war against the alien Covenant. The super-soldier Master Chief escapes a doomed battle-cruiser with the AI Cortana, crash-landing on a mysterious ring-world called Halo. While rallying surviving Marines, they discover Halo is an ancient weapon designed to sterilize all sentient life to contain the parasitic Flood. After rescuing Captain Keyes from Covenant torture, Chief learns the Covenant mistakenly worship Halo as holy. Fighting through Flood-infected corridors and Forerunner defenses, Chief activates Halo's self-destruct instead of its weapon, destroying the ring and the Flood with it. He escapes with Cortana, the only survivors.
