In a brief prologue set in 1958, two summer camp counselors at Camp Crystal Lake sneak away from a camp fire sing-along to have sex. Before they can completely undress, an unseen assailant sneaks into the room and murders them both.
The film flashes forward twenty-two years; a young girl named Annie enters a small diner and asks for directions to Camp Crystal Lake, much to the shock of the restaurant's patrons and staff. A strange old man whom the locals refer to as "Crazy Ralph" reacts to the news of the camp's reopening by warning Annie that they are "all doomed". A truck driver from the diner gives her a lift and warns her about the camp, informing her that in 1957, a young boy drowned in the camp's lake, one year before the double murders occurred. After the driver lets her out, Annie hitches another ride in a jeep. This second driver, whose face is never seen, ends up murdering Annie by slashing her throat with a large hunting knife.
Back at the camp, the other counselors (Ned, Jack, Bill, Marcie, Alice, and Brenda) are refurbishing the cabins and facilities along with the camp's new owner, Steve Christie. As a violent storm closes in on the horizon, Christie leaves the campgrounds, and the same killer who murdered Annie begins to kill off the counselors in gruesome ways, luring them into isolated areas and then mutilating them with various weapons. Later that evening, Christie returns from town and is also murdered, apparently familiar with his attacker.
Unaware that they are the last two left alive, Alice and Bill become suspicious and investigate, finding a bloody axe in one of the beds. They then discover the phones are dead, and the cars won't start. When the power generators fail, Bill goes to turn them back on and when Alice follows later, she finds his body gruesomely 'arrowed' to the back of a door.
Now alone, Alice flees back to the main cabin and hides. After a few moments of silence, Brenda's corpse is hurled through the window.
Alice hears a vehicle outside the cabin and, thinking it to be Christie, runs out to warn him. Instead she finds a middle-aged woman she has never seen before. She introduces herself as Mrs. Voorhees, an "old friend of the Christies", and Alice hysterically tries to tell her about the murders. Mrs. Voorhees expresses horror at the sight of Brenda's body, but her behavior is erratic, and she soon reveals herself to be the mother of the boy who drowned in the lake in 1957. Talking mostly to herself, it becomes clear that Mrs. Voorhees is the killer, and that the murders are her way of taking revenge for the death of her son, Jason. She blames his drowning on the fact that two counselors were distracted by having sex and were unaware of Jason's struggling in the lake. Mrs. Voorhees suddenly turns violent and pulls out a large knife, rushing at Alice.
A lengthy chase ensues, during which Alice flees her attacker and finds some of the bodies of the others in the process. After several violent confrontations, Alice finally manages to put Mrs. Voorhees down by striking her in the head with a cast-iron skillet. Believing her to be dead, Alice walks down to the lake's shore in a daze, but the insane woman lunges at her again with a machete. Alice disarms her, and in desperation, she decapitates Mrs. Voorhees with her own weapon.
The next morning, police arrive at the camp and find Alice floating on the lake, alone in a canoe. Just as she looks up at them, the monstrously misshapen body of a young boy, presumably the long-lost Jason, bursts out of the lake, pulling Alice into the water. In an ambiguous twist, Alice wakes up screaming in a hospital bed, unsure about whether the attack by Jason was real or imagined.
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