The game stars an unnamed marine, a survivor of Bravo Team during the events of Doom 3. He awakens, and is soon confronted by Dr. Garret, who presents him Sam, a flying droid capable of opening doors and hacking computers. Together with Sam, the survivor makes his way through the Mars facility, battling zombies and demons on his way through the base. The task is to reach a port, where a spaceship full of surviving members of the UAC facility is preparing to launch off. On his way, the marine meets more survivors, collects valuable information through Sam, and visits Hell, where he closes the demon-spawning portals. Eventually, the marine is forced to leave Dr. Garret behind, and Sam sacrifices itself to support a closing door to the spaceship, leaving only the data implicating UAC. The marine successfully boards the ship and leaves Mars with a few other survivors.