Halo 3: ODST happens mostly at the same time of the events of Halo 2 and the very beginning of Halo 3. It details the Covenant's invasion to Earth in year 2552 from the point of view of the Orbital Drop Shock Troopers. The player takes the role of various members of the ODST squad in the war-torn city of New Mombasa - most prominently The Rookie, who can be considered the protagonist of the game.
A departure from previous games in the Halo series, Halo 3: ODST is more open-ended and features more exploration. The new VISR visor system can provide light amplification and is used to identify friends and foes. The visor also provides map, navigation data, and link-up with the AIs that run the city. The players are also no cybernetic super-soldiers this time; player stamina and health are bigger factors than before, and both the speed and jump height are scaled back.
The game features a Firefight mode, a four-player cooperative game mode that is centered around the campaign. Rather than the hardcore sci-fi of previous editions, ODST has influences of film noir and jazz-like elements to complement the soundtrack.
Halo 3: ODST also includes Halo 3: Mythic, which is essentially a stripped-down, multiplayer-only version of Halo 3, and includes all Halo 3 downloadable maps, including Heroic, Legendary, and Mythic map packs and three previously unreleased maps.