On the way to California, a family has the misfortune to have their car break down in an area closed to the public, and inhabited by violent savages ready to attack.
An old man named Fred (John Steadman) is packing his truck. A ragged and somewhat feral teenage girl approaches. Annoyed, Fred addresses her as Ruby. She offers to trade what she has in her bag for food, but the old man refuses. They walk into a small cabin and Fred scolds her for what she and "they" have done. Ruby says that her family ambushed a nearby airfield because they were hungry and no one passes by their home anymore. She pleads with Fred to take her with him. He tells Ruby that if "the pack", in particular someone named Jupiter, learns what she is doing, she could be in danger. A noise distracts them, and Ruby hides.
The Carter family is traveling on vacation. Parents Bob (Russ Grieve) and Ethel (Virginia Vincent) are driving, accompanied by their teenage children Bobby (Robert Houston) and Brenda (Susan Lanier), eldest daughter Lynne (Dee Wallace), along with Lynn's husband Doug (Martin Speer), baby daughter Katie, and their dogs, Beauty and Beast. They stop at Fred's Oasis for fuel. Fred tells them to stay on the main road. Later, they skid off a desert road and crash, owing to what is later revealed to be a booby trap. Bob walks back to Fred's Oasis to get help. Fred's son and his son's family of deranged cannibals dwell in the wilderness through which the Carters are traveling. They are commanded by Papa Jupiter (James Whitworth), patriarch of the clan. He killed his mother, Fred's wife, during childbirth. As a child, he killed the livestock on his father's farm and later murdered his sister. Fred attacked his son with a tire iron and left him in the wilderness to die. Jupiter survived, and mated with a depraved, alcoholic prostitute known as Mama (Cordy Clark). Together, they had three sons, Mars (Lance Gordon), Pluto (Michael Berryman) and Mercury (Arthur King), and their abused daughter Ruby (Janis Blythe). They survive by stealing from and cannibalizing travelers.
As night falls, Bob reaches the gas station, where Fred tells him the origin of the hill people. Papa Jupiter arrives, kills Fred with a tire iron and takes Bob prisoner. Doug and Lynne are sleeping in the car while everyone else stays in the trailer. Bobby gets locked out of the trailer and asks Doug for his keys. Bobby does not know the trailer is locked because Pluto is looking through their valuables, while Ethel and Brenda sleep in the next room. As Bobby enters the trailer, Papa Jupiter sets Bob ablaze on a stake in the distance. Ethel, Lynne, Doug and Bobby rush to Bob, while Brenda stays in the trailer with the baby. Everyone tries extinguishing the fire, while Pluto and Mars rape Brenda. The Carters extinguish the fire, but Bob dies shortly after. When Ethel and Lynne return to the trailer, Pluto runs away. Lynne finds Mars taking her baby. She attacks Mars as Ethel hits him with a broom. Mars shoots Ethel and Lynne. Pluto abducts the baby and flees with Mars. Doug rushes in to find Lynne dead; Ethel dies shortly after.
Doug sets out to find his baby while Bobby and Brenda remain behind. Beauty has been savaged by the clan, and Ruby is forced to eat it as punishment. She is chained outside the cave where the clan live, with Mama tormenting her. The men of the clan return to the cave, but Mercury is ambushed and pushed to his death off a hilltop by Beast. Papa Jupiter and Pluto return to the trailer to kill the survivors. Pluto is incapacitated by Beast, and Papa Jupiter is killed by a trap set by Brenda and Bobby. Doug sees Ruby knock out Mama and escape with Katie into the hills, followed by Mars.
Doug catches up with Ruby and the baby. Mars follows them into the hills, but as he tries to kill Doug, Ruby interferes by putting a rattlesnake against Mars' neck, enabling Doug to overpower his assailant. The film ends with a closeup of Doug frantically stabbing Mars to death while Ruby weeps over her brother's body.