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.