The game begins as the Pillar of Autumn exits slip-space and its crew discovers a large ringworld structure of unknown origin. The Covenant pursues the Autumn and attacks. With the ship heavily damaged, the Autumn's captain, Jacob Keyes, entrusts the ship's artificial intelligence (AI) known as Cortana to Master Chief in order to prevent the Covenant from discovering the location of Earth. Keyes orders the crew to abandon the Autumn and pilots the ship to a crash-landing on the ringworld. On the ring's surface, Master Chief and Cortana rescue scattered survivors and help organize a counter-offensive against the Covenant.
The player can move around and look up, down, left, or right. The game features vehicles, ranging from armored 4×4s and tanks to alien hovercraft and aircraft, many of which can be controlled by the player. The game switches to a third-person perspective during vehicle use for pilots and mounted gun operators; passengers maintain a first-person view. The game's heads-up display includes a "motion tracker" that registers moving allies, moving or firing enemies, and vehicles, in a certain radius of the player.
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