In Halo 3: ODST players explore the ruined city of New Mombasa to discover what happened to their missing teammates in the midst of an alien invasion. In the “Firefight” multiplayer option, players battle increasingly difficult waves of enemies to score points and survive as long as possible; Halo 3’s multiplayer is contained on a separate disc packaged with ODST.

The game’s Campaign mode may be played alone or cooperatively with a maximum of three additional players. As a lone human soldier known as “the Rookie,” the player’s goal is to discover what happened to his missing teammates. After finding a piece of evidence left behind, such as a sniper rifle hanging from a power line, the player will enter a flashback mission, where they assume the role of the missing soldier six hours earlier. After the player has found the first piece of evidence, the choice of where to go next is left open; campaign levels may be played in any order.