Friday the 13th
In a brief prologue set in 1958, two summer camp counselors at Camp Crystal Lake, named Barry (Willie Adams) and Claudette (Debra S. Hayes), 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 moves forward to (the present) Friday June 13th; a young woman named Annie (Robbi Morgan) 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 named Ralph (Walt Gorney) reacts to the news of the camp's reopening by warning Annie that they are "all doomed". Enos (Rex Everhart), a friendly truck driver from the diner, agrees to give Annie a lift halfway to the camp. During the drive, he warns her about the camp, informing her that a young boy drowned in Crystal Lake in 1957, one year before the double murders occurred. After Enos lets her out, Annie walks on. A little later, Annie hitches a ride and gets picked up by an unseen driver in a Jeep. As the driver speeds past the entrance to the camp, Annie becomes concerned and leaps from the moving vehicle when the driver fails to stop, fleeing into the woods. The unseen driver chases her into the woods and slits her throat with a bowie knife.
At the camp, the other counselors, Ned (Mark Nelson), Jack (Kevin Bacon), Bill (Harry Crosby), Marcie (Jeannine Taylor), Alice (Adrienne King), and Brenda (Laurie Bartram) are refurbishing the cabins and facilities along with the camp's demanding owner, Steve Christy (Peter Brouwer). They have a few strange incidents, including a prank where one of them pretends to drown, and an unwelcome visit from the local police. They find a large snake in one of the cabins, chopping it in half with a machete. They also receive a visit from Ralph, who again warns them that they are "doomed". As a violent storm closes in on the horizon, Steve leaves the campgrounds to get more supplies.
Meanwhile, the unidentified killer (still unseen except for a pair of black slacks, a brown plad shirt and a class ring on his/her left ring finger) begins to isolate and murder the counselors one by one. Ned spots the killer (now wearing a hooded black rainslicker) walking into a cabin. Nick follows the person into the cabin.
As the violent rainstorm hits the camp, Marcie and Jack later enter the cabin and have sex in one of the bunks, unaware that Ned's dead body is in the upper berth. Afterwards, Marcie leaves to visit the ladies bathroom, and Jack is attacked by the killer, who is lying in wait underneath the cot. The murderer drives an arrow up through the bed and it pierces Jack's throat, killing him.
While using the bathroom, Marcie hears a noise by the showers. Thinking its Jack playing a joke on her, she goes and pulls back a shower curtain. Finding nothing, she turns around and finds the unseen killer wielding an Kelly Perfect-modeled felling axe and gets it slammed in her face.
Bill, Alice, and Brenda play "strip Monopoly" in Alice's cabin, until the rain makes Brenda remember she left her cabin windows open. As Brenda settles down to bed, she hears a faint, child-like voice calling "Help me!" from the woods. Brenda walks outside with a flashlight in the pouring rain trying to locate where the child's voice is coming from. She ventures to the archery range where the hooded rainslicker killer turns on the blinding lights at the range, distracting Brenda who is then killed off-screen (presumably by getting hit with an arrow or two).
Hearing her scream, Bill and Alice go to investigate but only find the bloody axe (the same axe used to kill Marcie) in Brenda's bed. After looking around the camp, they find nobody and begin to suspect that something is up. Attempting to call the police, they discover the phone lines have been cut and that the cars won't start when they try to leave.
Meanwhile, Steve is in town having dinner at a local diner when he goes back to his jeep for the drive back to Camp Crystal Lake. But during the drive, his jeep, towing a trailer, gets stuck in the mud off a secondary road. Yet a passing policeman gives him a ride part of the way back to the camp. Steve is let out as the officer gets a call about a car accident in town. With the rain letting up, Steve begins to walk back to the camp alone. When Steve arrives at the entrance to the camp, he is also murdered, apparently familiar with his attacker.
When the lights go out all over the camp, Bill goes to check on the power generator, but doesn't know how to operate it or figure out what is wrong. When Bill doesn't return, Alice heads out to the generator cabin looking for him, and she finds his body pinned to a door by several arrows.
Now alone, Alice flees back to the main cabin and hides. After a few moments of silence, Brenda's corpse is hurled through a window. Alice hears a vehicle outside the cabin and, thinking it to be Steve, runs out to warn him. Instead she finds a middle-aged woman who introduces herself as Mrs. Voorhees (Betsy Palmer), an "old friend of the Christys". Alice hysterically tries to tell her about the murders. Mrs. Voorhees expresses horror at the sight of Brenda's body, but she soon reveals herself to be the mother of the boy who drowned in the lake in 1957. Talking mostly to herself, she blames her son Jason's drowning on the fact that two counselors were having sex and were unaware of Jason's struggling in the lake. She tells Alice that today happens to be Jason's birthday... Friday the 13th. Mrs. Voorhees suddenly turns violent and pulls out her Sabre Monarch bowie knife (thus revealing the bowie knife as well as her class ring on her left ring finger confirming that Mrs. Voorhees is the killer), rushing at Alice. A lengthy chase ensues, during which Alice flees her attacker and finds Steve's and Annie's bodies in the process.
Mrs. Voorhees turns back on the generator to the camp and then chases Alice all over the camp in a climatic chase... sometimes speaking to herself in Jason's voice. She corners Alice in a storage shed, where Alice has found a shotgun. But before Alice can load it, Mrs. Voorhees attacks her and slaps her around. Alice escapes and runs back to the main cabin, hiding behind a locked door in the pantry. Mrs. Voorhees breaks the door open and rushes in at Alice brandishing a machete. Alice hits her on the side of the head with a cast iron skillet, rendering her unconscious and hopefully dead.
Heading out to the side of the lake, Alice sits down next to the water, but Mrs. Voorhees comes rushing out of the darkness at her again with the machete. Narrowly avoiding the machete, Alice knocks it out of her hands with a paddle and the two of them wrestle next to the lake. Besting her attacker, Alice grabs the machete and decapitates Mrs. Voorhees... killing her once and for all. In shock, Alice gets in a canoe and rows out to the middle of the lake, where she falls asleep.
Morning comes, and Alice is still asleep in the canoe. Police arrive and call out to her, and she awakens. As she sits up and contemplates her own rescue, the decomposing "corpse" of Pamela's son, Jason (Ari Lehman), attacks Alice and pulls her out of the canoe. Just as she is dragged underwater, she wakes in a hospital, where a police officer tells her that they pulled her out of the lake. When she asks about Jason, the officer informs her they never found any boy. Alice replies, "Then he's still there."
