ODST takes place in the 26th century, when humans under the command of the United Nations Space Command (UNSC) are locked in a war with a theocratic alliance of alien races known as the Covenant. During the events of the 2004 video game Halo 2, the Covenant discover the location of Earth and launch an assault on the city of New Mombasa in Africa.
Though the UNSC manages to repel most of the fleet, a large ship hovers over the city, depositing an invasion force. The ship eventually retreats via a slipspace jump, creating a massive shockwave. While the rest of Halo 2's storyline follows the ship to an ancient installation similar to the first Halo, ODST focuses on the aftermath of the shockwave, while the Covenant still occupy the city.
During the game, the player can unlock audio files to an additional narrative called "Sadie's Story", which tells the story of a civilian girl caught in the initial stages of the Covenant invasion and her quest to find her father. The tale can also provide useful information for the player during the game, such as helping to locate hidden caches of weapons.
The game's protagonist is the unnamed Rookie, a new member of a group of Orbital Drop Shock Troopers. Troopers, known as ODSTs or Helljumpers, often deploy in small, one-man Human Entry Vehicles (HEVs), referred to in-game as Single Occupant Exo-Atmospheric Insertion Vehicles (SOEIV), launched from spaceships in the upper atmosphere.