A group of adolescents gathered around a nighttime campfire at Crystal Lake listens to a young man recount the murderous activities of Jason Voorhees, who reportedly drowned as a boy, but now terrorizes visitors to the lake. Later, in the aftermath of Jason’s most recent slaughter, in which he was allegedly killed, police and ambulance personnel clean up the carnage. Jason’s corpse is transferred to the Wessex County Medical Center morgue. The attendant, Axel, seduces a female nurse named Morgan near the body. As the proximity to the corpse unnerves Morgan, she orders Axel to put it in the freezer, and returns to work. After placing the body in cold storage, Axel watches television and is killed by Jason, using a surgical saw. The hockey mask-wearing Jason then stabs Morgan to death in a supply room. Back at Crystal Lake, divorcee Mrs. Jarvis settles into a new home with her teenage daughter, Trish, and son, Tommy. Nearby, a group of adolescents in a station wagon bypass a young female hitchhiker. Moments later, she is stabbed through the neck from behind. Meanwhile, the teenagers move into the house next door to the Jarvises and meet Trish and Tommy. Later, Tommy watches one of the girls, Samantha, undress from his bedroom window. The following day, on a trip to Crystal Point, the teens meet uninhibited twins Tina and Terri and the group swims naked. Tommy witnesses the activity and Trish hurries him away, but their car breaks down. When a backpacker named Rob Dyer assists them, they bring him home, and Tommy proudly shows off his collection of homemade monster masks. That night, the teens throw a party, where Tina flirts with new friend, Paul. His girl friend, Samantha, becomes jealous and goes for a swim alone. In the water, she climbs into a rubber raft and is stabbed from below. Meanwhile, Paul rebuffs Tina’s advances and looks for Samantha. Onto her new conquest, Tina takes awkward Jimmy upstairs, leaving Terri with the more aggressive Ted. At the lake, Paul discovers Samantha’s body and is killed when he swims back to the dock. Rob hears Paul scream from his campsite and investigates. When he returns to his tent, his hunting rifle has been snapped in two. Ted discovers a silent stag film and watches it with Sara and Doug, but Terri becomes bored. When Tina refuses to abandon Jimmy, Terri leaves alone in the rain. As she prepares her bicycle, Jason impales her against the side of the house. Sara confides to Doug that she wants to make love with him and goes upstairs to get ready. After Mrs. Jarvis returns from jogging to find nobody home and the power out, she ventures back outside and is killed. Later, siblings Trish and Tommy return home and become concerned when their mother is not home. Trish tells Tommy to stay in the house, and she looks for their mother, but takes refuge from the rain in Rob’s empty tent, startling him. Meanwhile, Jimmy goes to the kitchen after making love with Tina and is killed by Jason. Later, Tina looks out an upstairs window as Jason, clinging to a trellis, reaches through the glass and throws her to the ground. She lands dead on the roof of the station wagon. Elsewhere, in the tent, Rob informs Trish that his sister was one of Jason’s victims and he is seeking vengeance. When Trish insists Jason is dead, Rob informs her that the body has disappeared from the morgue. Rob shows her newspaper clippings about Jason from his research. In the living room of the teenagers’ house, Ted is stabbed through the back. Upstairs, Doug showers after making love with Sara, and Jason crushes his skull. Sara discovers the body and as she flees, Jason plants an ax in her chest. Ron and Trish check on Tommy, then go to the teenagers’ house. As Rob explores the basement, Trish searches upstairs. Meanwhile, Tommy peruses Rob’s newspaper clippings about Jason. Trish finds Doug’s body, and when she goes downstairs to find Rob, Jason kills him. Armed with Rob’s machete, Trish runs home, where she and Tommy attempt to nail shut the windows. Jason throws Rob’s body through the window and grabs Tommy. Trish buries the claw end of a hammer in Jason’s head, allowing her and Tommy to escape upstairs. When Jason follows, Trish attempts to electrocute him by breaking a television over his head. She draws Jason away from Tommy by running back to the teenagers’ house. Upstairs, she dives through a window, rolls off the roof, and falls to the ground. Meanwhile, Tommy shaves his head to make himself look like Jason as a child. The killer pursues Trish back to the Jarvis house and she fights him off with the machete. Tommy appears on the stairs and calls to Jason. The killer is momentarily stunned by the boy’s appearance and Trish strikes him with the machete, knocking off his hockey mask. When he turns to attack Trish, she drops the weapon. Tommy picks it up and strikes Jason, the blade going deep into his skull, and the psychopath falls to the floor. As Trish embraces her brother, Tommy notices Jason’s fingers move and he strikes the body repeatedly. Later, in the hospital, the sheriff and a doctor console the wounded Trish. She inquires about Tommy, and the doctor assures her he will be all right, too, despite the trauma. The doctor allows Tommy to see her and they embrace, but he stares over her shoulder, disturbed.