Halo 3: ODST is a first-person shooter set in New Mombasa during the Covenant invasion. The campaign follows the Rookie, an Orbital Drop Shock Trooper, as he searches the city and reconstructs the events that separated his squad. The game includes the complete Halo 3 multiplayer map collection, with three additional maps, and introduces Firefight, a cooperative mode in which players face waves of Covenant enemies.

Taking place in the Kenyan port city of New Mombasa, ODST follows a squad of elite Marine Orbital Drop Shock Troopers during the Covenant invasion of Earth on October 20, 2552. Knocked off course due to the Mombasa Slipspace Incident, these five ODSTs, accompanied by an ONI officer, fight through the waste-filled, destroyed streets in order to link up with each other, evade the Covenant, and execute a top secret mission. The game occurs directly after the events of the Halo 2 level Metropolis, when the Prophet of Regret performs a slipspace jump in New Mombasa.