In the suburban Northeast, the Collingwoods live in a secluded house near a lake. On the day before her seventeenth birthday, Mari, the Collingwoods' only child, is planning to attend a rock concert in the city with her friend Phyllis Stone. Mari's father John, recalling that the band Blood Lust had killed a chicken onstage during a previous concert, reminds Mari that her generation is supposed to be about love and peace. As an early gift, Mari's parents present her with a necklace that has a peace symbol pendant. Before going into the city, Mari and Phyllis sit in the woods and share girl talk. On the drive, a radio broadcast announces the prison escape of Krug Stillo and Fred "Weasel" Padowski, vicious criminals who were aided in their jailbreak by Krug's illegitimate son Junior and an "animalistic" unknown woman, who kicked a German shepherd guard dog to death. Meanwhile, in the apartment where the four criminals are preparing to leave the state, the woman, Sadie, who has been reading about women's liberation, tells Krug and Weasel, both of whom are her sexual partners, that she will not "put out" for them until there are two more "chicks." Soon after, Mari and Phyllis arrive in the city and decide to try to buy some marijuana before going to the concert. They spot Junior, who Krug has turned into a heroin addict in order to control him, and ask him if he has any marijuana. Claiming that he does, Junior leads them to the apartment, where the girls are taken prisoner, after which Sadie, Krug and Weasel torment and rape them all night. During this time, the Collingwoods decorate their house for Mari's birthday party and bake a cake. Early the next morning, the gang puts the semi-conscious girls in the trunk of their car and drive out of the city. As they drive, they joke about the best sex crimes of the century. Concerned that Mari has not returned home, the Collingwoods summon the sheriff, who assures them that their daughter will probably return by supper and says that his deputy, Harry, is calling New York to locate them. Later, Harry, who is not very bright, reports that the morgues and police stations in the area have not turned up anyone fitting their descriptions. When the police leave, they do not bother checking into a strange car parked near the Collingwood mailbox, thus losing the opportunity to capture the gang, whose car, by coincidence, has broken down outside Mari's home. Unaware that they are in Mari's neighborhood, Krug has his cohorts drag the girls into the woods, where they brutally and sadistically humiliate and torment them. After whispering to Mari to run for help, Phyllis tries to create a diversion by running in another direction and is chased by Sadie, Krug and Weasel. Meanwhile, Mari tries to reason with Junior, who has been left to guard her, although he feels remorse for what has been happening. Saying she wants to be friends, Mari gives Junior her necklace and explains that they are near her home and that her father, a physician, can help him overcome his drug addiction. She then leads him by the hand toward her home. When the sheriff receives a call reporting that the escaped criminals were seen in the area and includes a description of the car, he realizes that the gang's car is parked in front of the Collingwoods’ residence. He and Harry race to the house, but, on the way, run out of gas. Out of breath, Phyllis manages to run to a graveyard, but is surrounded there by Krug, Weasel and Sadie, who take turns stabbing her, watch as she crawls in pain, then cut out her entrails. The three later catch up with Mari and Junior and, when Mari asks if Phyllis got away, they show her Phyllis' dismembered hand. The police continue on foot and try to stop passing cars, but one car is filled with young occupants who make crude gestures as they drive by, and, later, a driver whose truck is overloaded with chickens refuses to make room for the lawmen. Meanwhile, the criminals slash Mari and Krug rapes her, as the horrified Junior begs him to stop. When Krug has finished, Mari, in a daze, gets up and walks away, as the thugs, exhausted from their blood lust, watch. She throws up, prays "Now I lay me down to sleep," and slowly walks into the lake. Taking a gun from Weasel, Krug shoots Mari several times, then tells the others to get cleaned up and prepare to leave. By 7:00 p.m., the police are still on foot about ten miles from the Collingwoods' house. Meanwhile, the gang knocks on the Collingwoods' door, claiming to be salespeople with car trouble. As their phone line has been out of order all day and Mari took their only car, the Collingwoods offer to let them spend the night and get help for them in the morning. During the night, Junior, who is suffering from withdrawal and trauma, has nightmares and goes into the bathroom to vomit. Hearing him, Mrs. Collingwood tries to help and sees Mari's necklace around his neck. She then surreptitiously checks the contents of a suitcase the gang brought in, finds bloodied clothes and alerts John. After searching the woods, they discover Mari's body and bring it back to the house. As John retreats to the basement to determine what to do, Weasel awakens from a nightmare and finds Mrs. Collingwood alone in the dark. She hints that her husband is having sexual problems and manipulates Weasel into boasting he could make love to her with his hands tied behind his back. As John is laying trip wires and booby traps in the hall and other parts of the house, his wife lures Weasel outside and dupes him into allowing her to tie his hands behind his back. She then loosens the zipper and pretends to perform fellatio, but instead bites off his organ. Weasel's screams awaken Krug, and he and John begin to fight, working their way into the living room, where Mari's corpse is lying on the couch. Krug seems to have the upper hand, when Junior appears with John's rifle and, shaking, threatens to shoot his father. Krug tells him to point it at himself and "blow your brains out," repeating the words over and over until the emotionally fragile Junior kills himself. When Krug returns his attention to John, he finds the man has gone to the basement. Taking the gun, Krug shoots at John, but John tells him there was only one bullet in the chamber. Wielding a chainsaw, John pursues Krug throughout the house. Krug tries to shield himself with furniture, but the chainsaw cuts through it. While trying to escape from the house, Krug receives an electrical shock from the doorknob John had rigged up earlier in the evening. Meanwhile, Sadie has awakened and runs out the door, pursued by Mrs. Collingwood. They struggle in the grass, until Mrs. Collingwood is able to stab the younger women and throw her into a swimming pool. As John cuts through Krug with his chainsaw, the sheriff arrives and watches in disbelief.