The film begins somewhere in the rural southwest USA in a desert ghost town. At the town's sole gas station, called Fred's Oasis, an old man named Fred (John Steadman) is seen packing his truck and drinking alcohol, looking very worried. He frequently glances at the arid and barren landscape around him, and makes continual comments about "them" and what "they" would do to him if they ever found out he was escaping. Suddenly, a ragged and somewhat feral teenage girl turns up. Annoyed, Fred asks the girl at what she is doing there... addressing her as Ruby (Janus Blythe). She offers to trade what she has in her bag for food and the old man refuses. They move into a small cabin to talk and Fred scolds her for what she and "they" have done recently. Ruby says that her family ambushed a group of passing tourists at the nearby closed-down airfield because they were hungry and no one passes by their home anymore. She pleads with Fred to take her away with him, but he mocks her and says she could never live with normal people in an urban environment. He demands to know whether "the pack," in particular someone named Jupiter, knows what she is doing. He warns her of the danger she is in if Jupiter knows, and she retorts that her "Pa" would do the same to Fred if he knew he was getting away from town. A noise distracts them and Ruby hides.
A large stationwagon pulling a large trailer pulls into the filling station. They make up of Carter family on vacation. They make up of 65-year-old Bob Carter (Russ Grieve) a retired policeman and his homemaker wife Ethel (Virginia Vincent). They are driving the family car accompanied by their twin teenage children Bobby (Robert Houston), and Brenda (Susan Lanier), and eldest daughter Lynne (Dee Wallace), along with Lynne's husband Doug (Martin Speer) and their baby daughter Katy (Brenda Marinoff). They stop at Fred's Oasis for fuel and to allow themselves and the family's German Shepherd dogs, Beauty and Beast, to stretch. Bob asks Fred for directions to the nearby interstate for they are on their way to California to visit an old silver mine left behind by the late Bob's father. Fred tells them to be sure and stay on the main road.
Later, as the Carter family continue on their way, the road they are traveling on becomes an unpaved dirt road and soon they skid off the road and crash their station wagon (due to what is revealed to be a booby-trap). Bob leaves Bobby a pistol and heads back to Fred's Oasis on foot to get help. Doug heads off in the forward direction hoping to find someone. Ethel, Bobby, Brenda, and Lynne remain behind with the trailer and their car, which upon inspection has a broken axle.
When one of the family dogs, Beauty, runs off, Bobby chases after her, only to find the dog dead having been killed and disemboweled by unseen assailants who are spying on the Carter family from the nearby hills and communicating with each other by walkie-talkies. Bobby returns to the trailer, but keeps the information to himself. Doug returns soon after, telling them that the dirt road led to a dead end and that he found numerous abandoned vehicles in a large crater at the end of the desert road. When Bobby tries to use their CB radio to call for help, they recieve strange responses from the other end of the line.
As night falls, Bob has arrived back at the gas station, where Fred attacks him then tells him the origin of the hill people. It turns out that Fred's son, Jupiter, and Jupiter's family of deranged cannibals dwell in the barren wilderness through which the Carters are traveling. They are commanded by Papa Jupiter (James Whitworth), the patriarch of the clan. He was born a mongoloid monster and killed his mother, Fred's wife, at childbirth. As a child he was vicious and brutal, killing all the livestock on his father's farm, then eventually murdering his sister. Fred eventually reached a breaking point, attacking his son with a tire iron and leaving him in the wilderness to die. The young Jupiter survived, and began living with a depraved, alcoholic prostitute known as Mama. Together they had three sons, the vicious Mars (Lance Gordon), the bald and hideous thug Pluto (Michael Berryman), and the mentally retarded Mercury (Pete Locke) as well as their abused daughter Ruby. They survive in the desert by stealing from and cannibalizing all who cross their path.
Papa Jupiter arrives, beats Fred to death with a crowbar, and takes Bob prisoner. Back at the stranded vehicle, Doug and Lynne are spending the night in the family car while everyone else stays in the large trailer. Bobby gets locked out of the trailer and asks Doug for his set of keys. Bobby does not know that the trailer is locked because Pluto is inside looking through their valuables for food while Ethel and Brenda are asleep in the next room. As Bobby uses the keys to enter the trailer, Papa Jupiter sets Bob ablaze on a stake out in the near distance to create a distraction. Ethel, Lynne, Doug, and Bobby rush to Bob, while Brenda stays in the trailer with the baby. Everyone tries to help extinguish the fire, while Pluto reveals himself in the trailer and attacks Brenda. Mars jumps down from the top of the trailer, goes inside, and rapes Brenda. The rest of the Carters extinguish the fire, but Bob dies shortly after. When Ethel and Lynne return to the trailer Pluto jumps out and runs away. Lynne rushes inside to find Mars taking her baby. She attacks Mars as Ethel steps in and hits him with a broom. Mars pulls out a gun and shoots Ethel in the stomach. Brenda rushes in and throws Lynne a knife, which she uses to stab Mars in the leg until he shoots her in the stomach as well. Pluto returns to abduct the baby for a cannibalistic meal and he flees with Mars. Doug rushes in to find his wife dead and Ethel dies shortly thereafter.
Of the extended Carter family, only Brenda, Bobby, Doug and (somewhere) baby Katy remain. Doug sets out to find his baby with the surviving dog Beast's help, while Bobby and Brenda remain behind. Back at the cannibal's lair, Ruby is being made to eat the dead dog Beauty as a punishment for attemping to run away. She is chained outside the cave where the clan lives, and Mama torments her. The men of the clan return to the cave where Mercury is pushed to his death off of a hilltop by the Carters' other dog, Beast. Enraged, Papa Jupiter threatens Ruby and vows to avenge his son's death.
At dawn the next day, Papa Jupiter and Pluto hurry back to the trailer, vowing to kill the remainder of the Carter family. Doug, meanwhile, spies on the savages' camp. He sees Ruby knock out her Mama and escape with baby Katy into the hills, being followed by Mars who stayed behind to guard Ruby. Doug catches up with Ruby and the baby.
Meanwhile, Pluto and Papa Jupiter are on their way to the trailer to kill the rest of the Carter family. Pluto is attacked and incapacitated by Beast. Papa Jupiter runs to the trailer and avoids a trap set by Brenda and Bobby involving the dead Ethel in a lawnchair outside the trailer. They devise another booby-trap, which blows Papa Jupiter and the trailer up when he tries to break in. Bobby goes to check the burning wreckage and is attacked by the injured and badly burned Papa Jupiter. Brenda attacks him with a hatchet and Bobby shoots him to death, killing him at last.
Mars follows Doug and Ruby into the hills. Doug turns to confront his pursuer and a brutal fistfight begins with Doug and Mars punching and wrestling each other on the ground. Ruby snares a large rattlesnake nearby and uses it to bite Mars. Taking advantage of the distraction, Doug wrestles Mars' knife away from him and brutally stabbs Mars to death, thus saving the baby. The film ends with a close up of the distraught Doug, who is still stabbing and kicking the dead body of Mars, as Ruby weeps over her brother's body.