In a post-apocalyptic world, a small group of scientists, led by Hal (George Kennedy), is living underground after a chemical warfare incident, since dangerous mutants lurk aboveground. When the group rescues Karen (Yvonne Saa), a female survivor, from the surface, she discovers that one of the monsters has impregnated her -- and soon her vicious offspring arrives to terrorize the crew. Meanwhile, another pregnant female survivor is unsure whether the father is man or beast.