Halo 3’s story centers on the interstellar war between twenty-sixth century humanity, led by the UNSC, and a collection of alien races known as the Covenant. The player assumes the role of the Master Chief, a cybernetic enhanced super-soldier, as he wages war in defense of humanity, assisted by Marines as well as an allied alien race called Elites, which is led by the Arbiter. The game features vehicles, weapons, and gameplay not present in previous titles of the series, as well as the addition of saved gameplay films, file sharing, and the Forge map editor; a utility which allows the player to perform modifications to multiplayer levels.
Halo 3 grossed US$300 million in its first week. More than one million people played Halo 3 on Xbox Live in the first twenty hours. As of January 3, 2008, Halo 3 has sold 8.1 million copies, and was the best-selling video game of 2007 in the U.S. Overall, the game was very well-received by critics, with the Forge and multiplayer offerings singled out as strong features. By March 2009 more than 1 billion online matches had been played.