Thrust into the climactic conclusion of the original Halo trilogy. The plot follows the story of the Master Chief, a super-soldier known as a Spartan-II, as he battles against the Covenant alien alliance and confronts the ancient threat of the Flood, all while seeking to save humanity and the galaxy from destruction.
The game picks up immediately after the events of Halo 2, with the Master Chief crash-landing on Earth. The Covenant, led by the High Prophet of Truth, is launching a full-scale assault on humanity's last stronghold, Earth. The Master Chief, along with his AI companion Cortana, must rally the remaining forces of the United Nations Space Command (UNSC) to defend Earth and stop the Covenant from activating a series of Forerunner artifacts known as the Halo rings.