Jennifer Hills (Camille Keaton) is a short story writer who drives out of her home in New York City to an isolated cottage by a lake in the countryside of upstate New York to write her first novel. The arrival in town of the attractive and independent young woman attracts the attention of Johnny (Eron Tabor), the local gas station owner, and his two friends Stanley (Anthony Nichols) and Andy (Gunter Kleemann), two unemployed youths who hang around the gas station. Jennifer receives a grocery delivery from Matthew (Richard Pace), the local simpleton who is shy and apparently mentally retarded, and befriends him. Matthew is friends with the other three men and reports back to them about the beautiful woman he met, claiming he saw her breasts.
The next day, Stanley and Andy start cruising by the cottage in their speedboat and surreptitiously prowl around the house at night while keeping themselves hidden from view. One day, while Jennifer is relaxing in her canoe, they surprise her in their speedboat and tow her to shore. As she tries to escape, she's met by Johnny, while Matthew hides in bushes nearby. She realizes they planned her abduction so Matthew can lose his virginity. Jennifer puts up a fight by punching Johnny, but is chased by the men through the forest. Matthew refuses to have sex with her so Johnny rips off Jennifer's bikini and rapes her. They allow her to escape but track her down shortly afterward and Andy brutally rapes her anally. Matthew again refuses to participate. The four guys flee from the scene of the crime in their speedboat.
A little later, after Jennifer crawls back to her house they attack her again. Matthew finally rapes her after drinking alcohol, but says that he can not reach orgasm with the other men watching. The other men ridicule her book and rip up the manuscript, and Stanley sexually assaults Jennifer and rapes her too. When Jennifer scratches at Stanley's face, she is savagely beaten. She passes out on the floor covered in mud from the woods as well as her own blood. With that, the men leave the house. Johnny realizes she is a witness to their crimes and orders Matthew to go back and stab her to death. Matthew cannot bring himself to do it, so he dabs the knife in her blood and returns to the other men claiming he has killed her. The four then take off in their speedboat.
Over the next two weeks, a traumatized Jennifer pieces both herself and her manuscript back together. She silently sits around her house, chain-smoking, and avoids looking into mirrors to avoid seeing her maimed face now sporting some permanent scars and ugly bruises from her beating. She goes to a church and asks for forgiveness, vowing to kill all four of the men who violated her. One day, the men learn Jennifer has survived and they beat Matthew up for lying to them about killing her.
Jennifer then begins spying on her assailants first by following Matthew home from the local grocery store to his parents house, and the next day she follows Johnny home from the garage to his house where he has a wife and two young children.
One day, Jennifer calls in a grocery order knowing Matthew will deliver it. Hearing that it's from Jennifer's address, he takes the groceries, and a knife he steals from the grocery store butcher unsure of what Jennifer plans to do with him. At the cabin, Matthew arrives where Jennifer entices him to have sex with her under a tree. As he becomes oblivious to his surroundings, she strings a noose around his neck and hangs him. Matthews struggles, but is slowly strangled to death as Jennifer watches with a wicked-looking facial expression. Jennifer cuts the rope and drops the dead body in the river. She then pushes his bicycle into the river and the groceries he brought as well. Jennifer then phones the grocery store to report that Matthew never arrived with her order.
The next day, Jennifer drives over to the gas station and seductively collects Johnny from the place and entices him to join her in her car. She stops on the road halfway to her house where she gets out of the car, and points a gun at him, ordering that he remove all his clothing. The clearly misogynist Johnny insists the rapes were her fault because she enticed the men by parading around in revealing clothing. She feigns belief in Johnny's ludicrous rant, lowers her gun, and invites him back to her cottage for a hot bath.
While taking a bath together, Jennifer manually stimulates him in her bathtub. When Johnny says that Matthew has been reported missing, Jennifer says she killed Matthew. Johnny thinks she is joking. As he nears orgasm, Jennifer takes the butcher knife Matthew brought with him from its hiding place under the bathmat and severs Johnny's genitals. He screams in terror while bleeding to death. Jennifer calmly leaves the room and locks Johnny in from the outside. He dies from blood loss. Jennifer dumps Johnny's dead body in the basement and burns his clothes in the fireplace.
The following day, Stanley and Andy learn that Johnny is missing from his wife. They take their speedboat to Jennifer's cabin aware that she might have killed him and Matthew. Andy goes ashore with an axe looking for Jennifer, while she swims out to the boat and climbs aboard before Stanley realizes what she is doing. She pushes him overboard and Stanley has trouble staying afloat for he is not a good swimmer. Andy tries to attack her when she speeds past him in the boat, but she escapes with the axe. Andy swims out to rescue Stanley but Jennifer plunges the axe into Andy's back, killing him. She backs the boat up to Stanley who grabs hold of the motor to climb aboard, begging Jennifer not to kill him. Looking down on her former assailant with very cold eyes, Jennifer instead smirks at Stanley and quotes what he said while assaulting her: "Suck it, bitch!" and starts the engine, disemboweling him underwater. As Andy and Stanley's dead bodies sink beneath the surface of the water, the murderous Jennifer smiles slightly as she speeds away in the boat.