Doom has a science fiction/horror theme. A background story is given in the game's instruction manual; the rest of the story is advanced with short messages displayed between each section of the game (called episodes), the action as the player character progresses through the levels, and some visual cues.rnrnThe player takes the role of a nameless space marine (dubbed "Doom-Guy") who has been punitively posted to Mars after assaulting his commanding officer, who ordered his unit to fire upon civilians. The Martian marine base acts as security for the Union Aerospace Corporation (UAC), a multi-planetary conglomerate, which is performing secret experiments with teleportation by creating gateways between the two moons of Mars, Phobos and Deimos. The manual makes it clear that Phobos is considered by space marines to be the dullest assignment imaginable: "with no action for fifty million miles, your day consisted of suckin' dust and watchin' restricted flicks in the rec room." This all changes when the UAC experiments go horribly awry. Computer systems on Phobos malfunction, Deimos disappears entirely, and "something fragging evil" starts pouring out of the gateway, killing or possessing all UAC personnel.