A meteorite crashes to earth during a torrential thunderstorm, bringing with it a slimy, three-headed alien beast that craves human meat and gives birth to hundreds of eel-like spawn. The monster takes up residence in the basement of a family home, devouring the parents without anyone else's knowledge. The oldest son Pete (Michael Robert Coleman) is a science major in college, and his younger brother Charles (Charles George Hildebrandt) is a horror movie fanatic who loves to dress up like his favorite monsters and scare his visiting aunt and uncle. When an electrician arrives to check the fuse box, Charles follows him into the basement in costume for a little fun, but instead discovers the hideous alien and what's left of his mother's corpse. Meanwhile, two of Pete's friends come over for a study session, bringing along the remains of a dead alien spawn that they found on the side of the road. Baffled, they dissect it in an effort to establish its origin, but it seems to defy everything they've ever been taught in class. Their intellectual curiosity is abandoned when they find Pete's uncle being eaten by a swarm of alien spawn, and from there on it's a fight for survival against the monster.