Fireteam Osiris is deployed to the planet Kamchatka, occupied by Jul 'Mdama's faction, to retrieve Halsey amidst a battle between Covenant forces and Forerunner Prometheans. Halsey claims to have information on a series of devastating attacks on several human worlds. The team kills 'Mdama and retrieves Halsey. Elsewhere, the Master Chief leads Blue Team on a mission to secure a derelict research station; the arrival of a Covenant fleet prompts Blue Team to scuttle the station instead. The Chief receives a cryptic message from Cortana, directing him to Meridian, a Human colony world. John is ordered to return to Infinity, but he and Blue Team disobey orders and set out after Cortana.
Players assume the role of powerful Spartan soldiers with a host of abilities. Revamped movement systems allow players to sprint indefinitely, though their damage-absorbing shield will not recharge; at top speed, the player character can slide across the ground or charge into an enemy. A thruster pack allows for a speed boost or lateral movements, and players can clamber up ledges to reach higher ground. Every weapon can be aimed while zoomed in, similar to iron sights; sustaining fire will knock players out of the zoom. By zooming while airborne, the player can briefly hover; while aloft, players can target the ground for an area-of-effect attack. The game features new weapons, as well as returning ones with tweaked mechanics.
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