A large family is headed to San Diego, California from Cleveland Ohio along with an infant baby and their two dogs Beauty and Beast. They are traveling in Ethel and her husband Bob Carter's RV. They stop at a gas station and are given advice on the area and where to site see by the gas station attendant. As the family continue their trip their tires are shredded when they drive over a hidden barb belt. After Bob takes off to a patrol station to get a tow truck, Beauty takes off into the hills after something. Bobby, Bob and Ethel's younger son, tries to chase down the dog. When he finds her she's dead. Scared and very upset about his dog he runs off but slips and knocks his head so heard he is left unconscious.
Inside the hills are many caverns full of mutants. One mutant named Ruby is intrigued by Bobby and investigates him while he is unconscious. Her brother starts to chew on beauty;s lifeless body while she looks at Bobby's features which are very different from her own.
When Bobby he awakes and returns to his family keeping quiet about Beauty. While asleep one of the mutant's enter the RV and tries to rape Brenda, Bobby's older sister. The family realizes that Bob is tied to a tree and set on fire and when they untie him it is too late. The mutants kill almost everyone except Bobby Doug, and Baby Catherine.
The rest of the movie follows Bobby and Doug as they find a way to rescue the baby. They go through killing the mutants one by one. Doug is told by one of them how the mutants came to be. They were caught in the middle of a nuclear testing area and mutated because of the radiation.
Ruby tries to return Catherine several times to dog but Lizard, another mutant, tries numerous times to stop her from returning the infant. Eventually Ruby decides to had the baby back and sacrifice her own life by tackling Lizard of the edge of a cliff to save Doug, Brenda, Bobby, and Catherine.