A hardened marine condemned to Mars for an act of conscience—refusing to gun down unarmed civilians—finds himself thrust into humanity's darkest hour when interdimensional experiments conducted by the Union Aerospace Corporation spiral catastrophically out of control. Demonic forces pour through unstable teleportation gates, obliterating the research stations on Phobos and Deimos and transforming survivors into shambling undead. Dispatched to secure the perimeter during a rescue mission, the player soon discovers that he alone has walked away alive, leaving him stranded on an overrun moon base with no escape route. His only path to survival lies through the heart of the infested complex, where he must fight his way past legions of hellspawn to find a way off Phobos.