In the not-so-distant future, a portal has been discovered leading from Earth to Mars, where the government operates a research base to examine the ruins of a lost civilisation; scientists are studying it, conducting experiments they perhaps shouldn't be, and trying find out what happened. Answer: nothing good. That's what Sarge (The Rock) and his unit of high-tech Marines find out the hard way when they're sent to investigate a disturbance on the base which turns out to be nothing less than an infestation of terrifying, evil, flesh-eating monsters.