Two months after the murders at Camp Crystal Lake, sole survivor Alice Hardy is recovering from her traumatic experience. In her apartment, when Alice opens the refrigerator to get her cat some food, she finds the severed head of Pamela Voorhees in her refrigerator and is murdered by an unknown assailant with an ice pick to her temple.
Five years later, Paul Holt opens a school for camp counselors on the shore of Crystal Lake. The camp is attended by Sandra Dier, her boyfriend Jeff Dunsberry, troublemaker Scott Cheney, tomboy Terry McCarthy, wheelchair-bound Mark Jarvis, sweet-natured Vickie Perry, jokester Ted Bowen, and Paul’s assistant Ginny Field, as well as many other trainees. Around the campfire that night, Paul tells the counselors the legend of Jason Voorhees, a boy who drowned at Camp Crystal Lake in 1957, prompting his mother to exact revenge on the counselors and commit another series of murders in 1979 to prevent the camp from being reopened, until she was killed by the last survivor; however, Jason is rumored to have survived and is now living in the woods near Crystal Lake, killing anyone who comes across him to avenge his mother’s death. After Paul finishes the story, a man with a spear scares everyone, but it’s only Ted in a mask. Paul reassures everyone that Jason is dead and that Camp Crystal Lake is now condemned and off limits.
That night, Crazy Ralph wanders onto the property to warn the group but is garroted from behind a tree by an unseen killer. The following day, Jeff and Sandra sneak off to Camp Crystal Lake and find a dog carcass before getting caught by Deputy Winslow and returned to the camp. Later, Winslow spots a masked man in a burlap sack running across the road and chases him into the woods, where he finds a shack before the man kills him with a hammer claw.
Back at camp, Paul offers the others one last night on the town before the training begins; six stay behind, including Jeff and Sandra, who are forced to stay as punishment for sneaking off. At the bar, Ginny muses that if Jason were still alive and had witnessed his mother’s death, it may have left him with no distinction between life and death, or right and wrong. Paul dismisses the idea, proclaiming that Jason is nothing but an urban legend. Meanwhile, the assailant appears at the camp and kills the counselors, one by one. Scott has his throat slit with a machete while caught in a rope trap, and Terry is killed off-screen upon finding his dead body. Mark gets the machete slammed into his face and falls down a flight of stairs. The killer then moves upstairs and impales Jeff and Sandra with a spear as they have sex, and then stabs Vickie with a kitchen knife.
Later, Ginny and Paul return to find the place in disarray. In the dark, the killer ambushes Paul and continues to chase Ginny throughout the camp and into the woods, where she comes across the shack. After barricading herself inside, she finds an altar with Pamela Voorhees’ severed head on it, surrounded by a pile of bodies. Realizing that Jason Voorhees is the killer, Ginny puts on Pamela’s sweater and tries to psychologically convince Jason that she is his mother. The ruse works for a time, until Jason sees his mother’s head on the altar and resumes his attack on Ginny. Paul appears and attacks Jason, but he is quickly overwhelmed. Just as Jason is about to kill Paul with a pickaxe, Ginny picks up the machete and slams it down into Jason’s shoulder, seemingly killing him.
Paul and Ginny return to the cabin and hear someone outside. Thinking that Jason has followed them, they open the door, only to find Terry’s dog, Muffin. Just as they sigh in relief, an unmasked Jason bursts through the window from behind and grabs Ginny. She then awakens to being loaded into an ambulance, and calls out for Paul, who is nowhere to be seen, leaving his fate ambiguous. Back in the shack, Pamela Voorhees’ head remains on the altar, with Jason nowhere to be found.