In the future, the player character (an unnamed space marine) has been punitively posted to Mars after assaulting a superior officer, who ordered his unit to fire on civilians. The space marines act as security for the Union Aerospace Corporation's radioactive waste facilities, which are used by the military to perform secret experiments with teleportation by creating gateways between the two moons of Mars, Phobos and Deimos. Three years later, Deimos disappears entirely and "something fraggin' evil" starts pouring out of the teleporter gateways, killing or possessing all personnel. The Martian marine unit is dispatched to investigate, with the player character left to guard the perimeter with only a pistol while the rest of the group proceeds inside the base and is killed. Being unable to pilot the shuttle off of Phobos by himself, he realizes that the only way to escape is to go inside and fight his way through the complexes of the moon base.