In DOOM, you're a space marine, one of Earth's toughest, hardened in combat and trained for action. Three years ago you assaulted a superior officer for ordering his soldiers to fire upon civilians. He and his body cast were shipped to Pearl Harbor, while you were transferred to Mars, home of the Union Aerospace Corporation. 

The UAC is a multi-planetary conglomerate with radioactive waste facilities on Mars and its two moons, Phobos and Deimos.  With no action for fifty million miles, your day consisted of suckin' dust and watchin' restricted flicks in the rec room. 

For the last four years the military, UAC's biggest supplier, has used the remote facilities on Phobos and Deimos to conduct various secret projects, including research on inter-dimensional space travel.

So far they have been able to open gateways between Phobos and Deimos, throwing a few gadgets into one and watching them come out the other. Recently however, the gateways have grown dangerously unstable. Military "volunteers" entering them have either disappeared or been stricken with a strange form of insanity--babbling, vulgarities,  bludgeoning anything that breathes, and finally suffering an untimely death of full-body explosion.  Matching heads with torsos to send home to the folks became a full-time job.  Latest military reports state that the research is suffering a small setback, but everything is under control.

A few hours ago, Mars received a garbled message from Phobos. "We require immediate military support.  Something fraggin' evil is coming out of the gateways! Computer systems have gone berserk!" The rest was incoherent.  Soon afterwards, Deimos simply vanished from the sky. Since then, attempts to establish contact with either moon have been unsuccessful. 

You and your buddies, the only combat troop for fifty million miles were sent up pronto to Phobos.  You were ordered to secure the perimeter of the base while the rest of the team went inside.  For several hours, your radio picked up the sounds of combat: guns firing, men yelling orders, screams, bones cracking, then finally silence.  Seems your buddies are dead. 

Things aren't looking too good.  You'll never navigate off the planet on your own.  Plus, all the heavy weapons have been taken by the assault team leaving you only with a pistol.  If only you could get your hands around a plasma rifle or even a shotgun you could take a few down on your way out.  Whatever killed your buddies deserves a couple of pellets in the forehead.  Securing your helmet, you exit the landing pod.  Hopefully you can find more substantial firepower somewhere within the station.  As you walk through the main entrance of the base, you hear animal-like growls echoing throughout the distant corridors.  They know you're here.  There's no turning back now.