Halo 3: ODST is a spin-off of the Halo series of futuristic shooters.
The game is set in the distant future when the humanity is at war with an alien empire, the Covenant. Unlike most Halo games, ODST doesn't feature the series' trademark protagonist Master Chief. In this installment, the main characters are the Orbital Drop Shock Troopers (the titular ODST). They do not possess the Master Chief's superhuman strength and power armor.
In the Campaign mode, the main protagonist is Rookie who only recently joined ODST. The characters are voiced by the actors of Firefly and Battlestar Galactica fame, such as Nathan Fillion, Alan Tudyk, and Tricia Helfer. At the beginning of the game, ODST soldiers prepare to defend the city of New Mombasa from a Covenant battleship.
ODST features an open world designed as a futuristic version of Mombasa, a city in Kenya. ODST do not have Spartans' abilities and equipment, they are weaker, slower and lack the deflector shields to protect them. Instead, Halo 3: ODST introduces the stamina meter that decreases when the character is hit and then slowly regenerates. When stamina meter reaches zero, the protagonist will lose health which does not regenerate.
ODST includes Halo 3 multiplayer modes and maps. The game introduces three more maps, a map editor and a new cooperative mode called Firefight. The campaign mode also allows up to four players to cooperate and take the roles of the other ODST soldiers.