The game begins immediately after the events of Halo 2 . The Master Chief (the protagonist) crashes into the African jungle on Earth after jumping from the Covenant holy city, High Charity . He is retrieved by the Arbiter (a former enemy turned ally) and Sergeant Johnson.

Carlos, whose technical know-how was just enough to be dangerous, ignored the warning signs. He clicked. A file named Halo_3_PC_Setup.exe began downloading. It was only 1.2 GB—far smaller than the 6.5 GB a real game would be, but he convinced himself it was "super compressed."

For many years, Halo 3 was not available on PC. This created a market for unauthorized fan ports. However, the official release of The Master Chief Collection on PC has rendered these obsolete and unnecessary for legitimate players.