**This synopsis is regarding the original cut of the film, not any of the various re-edits**
The film opens as we see a man feeding a corpse to a pen full of squealing pigs. This man is Zambrini (Marc Lawrence). He talks to the corpse as he prepares to toss it into the pen, explaining that the pigs were roaming free one night and they happened upon a drunk sleeping in a field. After eating the sleeping drunk, the pigs gained a taste for human flesh.
A Volkswagen bug driven by a young woman down a series of increasingly isolated roads. This is Lynn (Toni Lawrence), and the song that plays over the opening credits advises her to "keep on driving till the end of the road", because "somebody's waiting for you". After driving past a series of oil wells in rural California, she stops at a dead-end road, where she finds a diner. There is a sign in the window that says "Waitress wanted". As she steps out of her car, she is startled by the screaming of pigs, but she continues inside, where she meets Zambrini. This is his diner, and without much dialogue passing between them, he hires Lynn as a waitress and helps her get settled into a spare room where she will stay. She finds a strait razor in the medicine cabinet while she is preparing for bed. She has a nightmare that Zambrini enters her room and slashes her to death, but she wakes and realizes it was just a dream. She goes outside and starts looking around the property and hears the pigs, but before she can approach the pen, Zambrini grabs her and warns her never to go looking around back there.
A pair of older sisters are Zambrini's closest neighbors, Miss Macy (Catherine Ross) and Annette (Iris Korn). They are in the habit of calling the local sheriff, Dan Cole (Jesse Vint), and complaining about Zambrini's pigs. They can hear the pigs squealing from where they live, but they also claim that the pigs run free at night and come right up to their house. They claim Zambrini victimizes people by feeding them to his pigs, after which another pig will appear in the pen.
While Lynn is serving customers at the diner, a customer named Ben (Paul Hickey) begins hitting on her, although she rebuffs his advances. He tells her Zambrini's back story, about how he fell from a high wire during a performance and was pronounced dead, only to be taken to the morgue, where they discovered he was not dead at all. Dan Cole arrives and talks to Zambrini about the complaint he got from the Macy sisters, warning him to make sure his pigs are penned. He also meets Lynn, frightening her for a moment by telling her she is driving a "missing car". It turns out he's only referring to her registration, which is outdated. She promises to update it. Afterwards, Lynn makes a seemingly desperate phone call to her father, telling him that she had to leave but that she will return to be with him as soon as she can. We only hear Lynn's side of this conversation.
Later, Zambrini pays an uninvited visit to the Macys, entering their home unannounced and terrifying them by wearing bizarre makeup that he used during his days as a circus performer.
Ben returns to the diner and asks Lynn out again, this time telling her that he discovered an out of place nurse's uniform discarded in a field nearby, seemingly implicating Lynn, the stranger who has just arrived in town. Although Lynn tries to deny any connection, she agrees to go on a date with Ben. Unfortunately, Ben drives her to an isolated location and tries to force himself on her, seemingly willing to rape her if she doesn't comply. She is rescued by Sheriff Cole, who arrives and gives her a ride back to the diner.
Lynn mysteriously invites Ben back to her room at Zambrini's, apologizing for not accepting his advances. She puts on a show for him by wearing sexy lingerie, and then she gets into bed with him. However, she produces the strait razor and violently slashes Ben to death. Zambrini discovers the scene, with Lynn staring off into nowhere and singing to herself next to Ben's dead body. Calming Lynn, he cleans her up and takes her to his own room, where he puts her to bed. Afterwards, he removes Ben's body and feeds it to the pigs behind the diner.
Lynn wakes up the next day with seemingly no memory of the incident, but she walks down the road to a payphone, where she places another one-sided phone call to her father. When she can't reach him, she runs back to the diner, "chased" by her half-remembered screams of Ben and the threatening squeals of pigs.
Ben is missed, and Sheriff Cole begins investigating his whereabouts, leading him to the farm. He talks to the Macy sisters, who claim to have seen him arrive at Zambrini's, never to have left, while the pigs "went crazy". Dan questions Zambrini again and tries to look at the pig pen, attracted to it by the howling of Ben's dog, which has apparently located Ben's body by sense of smell. A human hand is lying just outside the pen, but Zambrini manages to hide it by standing on it, and Cole leaves. Dan returns to the Macys, where a doctor (Walter Barnes) is doing a checkup on one of the sisters. Dan and the doctor have a conversation about Miss Macy's belief that Zambrini turns people into pigs by feeding them to the swine, and the doctor tells him that ancient Egyptians actually believed this was possible, and that a human turning into a pig was a step toward becoming divine. Lynn herself proves impossible for Dan to question, staring off into space again and playing with her hair.
Cole isn't the only one looking for Ben, either. Zambrini discovers a group of young men prowling around his property, presumably Ben's coworkers at one of the oil wells. They find nothing, but beat Zambrini and tell him to leave town.
Things start to unravel for Lynn when a man named Jess Winter (Jim Antonio) arrives in town and starts asking for Lynn. He is eventually referred to the diner, where he approaches her in a non-threatening way and tells her they have some "mutual friends" who care about her. Our suspicions about Lynn are confirmed when he implies that Lynn has run away from a mental facility. He tells Lynn that she's welcome to come back, and Lynn is happy at first, asking about a doctor that she seemed attached to in a fatherly way. Zambrini overhears, and when Lynn goes to pack her things in order to return with him, Winter talks to Zambrini and tells him Lynn ran away from an insane asylum. When Zambrini asks about Lynn's father, he tell him Lynn has no father.
Having become attached to Lynn, Zambrini doesn't want her to leave, and he asks her to stay. Lynn agrees, luring Winter into turning his back on her and then stabbing him to death with a large kitchen knife. The body is dragged to the pig pen, where the pigs consume it.
Lynn's world continues to close in on her when the mental hospital calls the sheriff, telling him that Winter is missing and that Lynn is an escaped mental patient with homicidal tendencies. Dan calls Zambrini at the diner and tries to warn him about Lynn, telling him to act as if nothing is wrong and wait for them to come and take Lynn into custody. Zambrini, however, immediately tells Lynn the police are coming for her and that they need to make it seem as if she's left. When Lynn mentions her father, Zambrini repeats what the doctor told him: her father is dead. Lynn reacts violently and stabs Zambrini in the back, killing him.
Knowing the police are coming for her, she makes one final phone call to her "father", and this time we hear the recorded message on the other end telling her that she's reached a disconnected number. Lynn thinks she's talking to her father, telling him she loves him. While she's talking, the flesh-eating pigs from the pen suddenly enter the diner and advance on her.
Cole arrives at the diner and, presumably, finds the aftermath of the pigs eating the bodies of both Lynn and Zambrini. He later tells Ben's coworker that Lynn murdered her own father after enduring years of sexual abuse from him. The pigs are loaded into the back of a truck and taken away, but Dan is startled to find that instead of the 12 pigs he'd counted on previous visits to the farm, there are now 13 pigs. Also found in the pen is the Egyptian ankh pendant that Lynn has worn throughout the entire film.