At a run-down gas station in the middle of the desert, an old man, Fred (John Steadman), is packing his truck when a ragged and somewhat feral teenage girl named Ruby (Janus Blythe) approaches him. 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 "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, but he refuses and warns Ruby that she could be in danger if "the pack", in particular someone named Jupiter, learns what she is doing. She replies that his life will be in danger as well if Jupiter finds out that he is trying to leave. A noise distracts them, and Ruby hides.
The Carter family is traveling on vacation, towing a travel trailer from Ohio to Los Angeles; parents Bob (Russ Grieve) and Ethel (Virginia Vincent) are driving, accompanied by their teenage children Bobby (Robert Houston) Brenda (Susan Lanier), eldest daughter Lynne (Dee Wallace), Lynne's husband Doug (Martin Speer), their baby daughter Katie, and their dogs, Beauty and Beast. They stop at Fred's Oasis for fuel, and Fred urges them to stay on the main road as they leave. Fred's truck suddenly explodes, preventing him from leaving. Ignoring Fred's warning, the Carters skid off a desert road and crash. Bob walks back to Fred's Oasis to get help. The dogs become very panicky and start barking at the hills. Beauty runs off into the hills, where she is attacked by someone. Bobby chases after her, and finds her mutilated body. Frightened, he runs and falls, knocking himself unconscious.
As night falls, Bob reaches the gas station, where he finds Fred trying to hang himself. Fred tells him his son's family of deranged cannibals live in the hills through which the Carters are traveling. They are commanded by Papa Jupiter (James Whitworth), who 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 hills to die. However, Jupiter survived, and had children 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 an abused daughter, Ruby. They survive by stealing from and cannibalizing travelers. Papa Jupiter suddenly arrives and takes Bob prisoner, after killing Fred with a tire iron.
Brenda finds Bobby, who is shaken up about Beauty, and returns to the trailer but doesn't mention Beauty's death, not wanting to frighten the rest of the family. He gets locked out of the trailer and asks Doug for his keys, unaware the trailer is locked because Pluto is looking through their valuables, while Ethel and Brenda sleep. Before Bobby enters the trailer, Pluto signals Papa Jupiter to set Bob on fire on a stake in the distance. Ethel, Lynne, Doug, and Bobby rush to save Bob, while Brenda stays in the trailer with the baby. As they try to extinguish the fire, Pluto and Mars ransack the camper, and Mars rapes Brenda. The Carters eventually extinguish the fire, but Bob dies shortly afterward. When Ethel and Lynne return to the trailer, Lynne is attacked by Mars, and Ethel hits him with a broom. After Mars shoots Ethel and Lynne, Pluto abducts the baby, and the brothers flee. Hearing their screams, Doug and Bobby rush in the camper to find Lynne dead and Ethel mortally wounded.
The men of the clan return to the cave, but Beast pushes Mercury off a hilltop, and he falls to his death. Ruby is chained outside the cave, with Mama tormenting her, and is forced to eat Beauty as punishment for her betrayal.
The next morning, shortly after Ethel dies, Doug sets out to find his baby, while Papa Jupiter and Pluto set out to kill the survivors. Beast tears Pluto's throat out, and Papa Jupiter is killed by a trap set by Brenda and Bobby using their mother's corpse. Doug sees Ruby knock out Mama and escape with Katie into the hills.Doug catches up with Ruby and the baby, but Mars follows them. As he tries to kill Doug, Ruby interferes by putting a rattlesnake on Mars' neck, enabling Doug to overpower him, and Doug frantically stabs Mars to death while Ruby weeps over her brother's body.