The night after the events of Friday the 13th Part III, police and paramedics are cleaning up the mess left at the lakeside property of Higgins Haven by psychopathic killer and hideously deformed Jason Voorhees, who brutally slaughtered all but one of a group of teenagers visiting the property.
After being hit in the head with an axe by sole survivor of the murders, Chris Higgins, Jason is taken to the Wessex County morgue to be examined by the attending mortician, Axel.
Axel fails to notice signs that Jason is breathing. Jason arises and escapes escaping cold storage.
He murders Axel by slicing his throat with a surgical hacksaw and breaking his neck; and a nurse named Morgan, who is gutted with a scalpel. Jason leaves the morgue, and heads back to Crystal Lake.
The next day, a group of teenagers have rented a house on the shore of Crystal Lake and are headed to their weekend getaway. The teenagers include Paul, his beautiful, brunette girlfriend Samantha, the romantic couple Doug and Sarah, jokester Ted, and a hopeless romantic Jimmy.
Next door to the teenager's rental home, is another house where the Jarvis family resides: the mother, Mrs. Jarvis, her teenage daughter Trish, her pre-teen son Tommy (who likes to make horror masks), and the family dog Gordon, a golden retriever.
On the way to the rental house, the teenagers drive past Mrs. Voorhees's tombstone in a cemetery and a female hitchhiker, who they ignore.
The hitchhiker, rejected, sits back down on a log and begins to peacefully snack on a banana... until she is quickly murdered by Jason, who plunges a knife into her neck.
The teenagers arrive and meet Trish and Tommy and then later, meet two beautiful twin girls that live in the area, Tina and Terri.
The group all goes down to the lake to skinny dip. Tommy and Trish, on their way back from town, stop and while Tommy is fascinated, Trish struggles to shield Tommy's vision.
The teenagers, carefree and having a good time, invite Trish to a party later that night. Tommy and Trish continue driving on and a little ways down the road, their car breaks down.
They are soon assisted by a hitchhiker named Rob, who explains he is out hunting bears, but Tommy is quick to retort, claiming there are no bears in the area, and Rob quickly refutes, even though Tommy still finds his reasons for being here suspicious.
After helping with their car, Trish invites Rob back to the Jarvis house, where Tommy shows Rob several masks he had made himself before he leaves to go camping.
Night falls, and the teenagers (including Tina and Terri) throw a party. Meanwhile, Jason begins his bloody decimation of the group and Rob leaves the Jarvis house and pitches a tent in the nearby woods, as Tommy and Trish leave into town again.
Samantha grows jealous when Paul begins to dance with one of the twins, and leaves for the lake.
She starts skinny dipping and swims out to a raft, but her relaxation is ended quickly when Jason stabs her through the stomach with a knife from underneath the raft. Paul goes out to check on Samantha, and swims out to the raft, only to find that she is dead. In horror, Paul swims back to shore, only for Jason to lunge out from the water, and shoot a spear into Paul's groin, killing him in a cringe-inducing, yet well-known, death scene.
At the Jarvis house, Mrs. Jarvis returns home to find the power out, and when she goes out to investigate she too is killed.
Jimmy gets Tina to have sex with him, and Ted, Doug and Sarah begin watching some old vintage stag films. Terri tries to leave early on her bicycle but Jason stops her from leaving and stabs her in the back with a spear, killing her.
Tommy and Trish return home to find their mother missing and Jason gains entry to the rental home. Jimmy finishes with Tina, and goes into the kitchen to look for a corkscrew. But it seems Jason has found it first and stabs the corkscrew into Jimmy's hand and then buries a meat cleaver into his head.
Tina begins to get dressed and goes to look out of the bedroom window, only for Jason (who is standing on a ledge beside the window) to smash through the window, grab Tina, pull her out and throw her to the ground, and she lands on the roof of the teenager's parked car, killing her. Trish has gone to look for her mother and finds Rob, who tells her the real reason why he is here. Jason murdered his younger sister Sandra (killed along with her boyfriend Jeff while they were having sex in Friday the 13th Part 2) and warns Trish that Jason may still be alive.
Trish and Rob run back to the house to check on Tommy. Back at the rental home, Jason makes his way through the house and Ted becomes his next victim, when Ted gets too close to the projector screen he is watching his movies on and Jason stabs a knife through the projector screen...and into Ted's skull.
Doug and Sarah are upstairs, having sex in the shower while all of this is going on, and Sarah leaves to get dressed. Jason sneaks into the bathroom and kills Doug by smashing his head against the shower wall.
Sarah returns to find Doug dead, and runs downstairs where she runs into the front door, which is locked. A thrown axe bursts through the door and hits her in the chest, killing her. Trish and Rob return to the Jarvis house and see that Tommy is OK. They rush over to the rental house to check on the teenagers, along with Gordon, as Tommy stays back at the house. Trish and Rob enter the rental house to find it empty. Rob and Trish go down to the basement.
Gordon, frightened by something or someone, flees upstairs and escapes through a window. Trish runs upstairs to find Gordon, and finds Doug's corpse. She runs to the basement to warn Rob, but it is too late. Jason attacks Rob and kills him, and Trish flees, with Jason in pursuit. Trish, armed with Rob's machete, escapes the rental house and runs back to the Jarvis House, and Tommy and Trish nail the front door shut. After a few moments of silence, Jason hurls Rob's dead body through a window and smashes through another window to grab Tommy.
Trish narrowly manages to get Tommy away from Jason after striking him in the neck with a hammer claw.
The killer bursts through the door and throws the hammer at Trish, which misses her head by a few inches and sticks to the wall. Trish and Tommy run upstairs and lock themselves in Tommy's bedroom.
Jason begins to hack the door with an axe, Trish snatches up Tommy's computer monitor and smashes it on Jason's head, rendering him unconscious. But Jason quickly recovers and chases Trish out of the house and all the way to the rental house.
Meanwhile, Tommy finds some of the newspaper clippings Trish saved from Rob, and notices a picture of a bald Jason as a child. Tommy devises a plan and begins to shave his head bald.
Trish runs from Jason through the rental house and manages to escape by leaping through a second-story window. She quickly returns to the Jarvis house, and struggles with Jason - with two quick machete strikes, she splits in half his left hand and pierces through his chest.
Tommy runs downstairs, his head completely shaved, and distracts Jason, who falls into a brief trance, transfixed with Tommy who shares a resemblance with Jason when he was a child.
During this, Trish picks up the discarded machete and swings the machete at Jason's face, not wounding him, but knocking off the mask and revealing his hideous deformities.
Horrified, Trish drops the machete and Tommy grabs it and swipes it into Jason's head. The killer drops to the ground and falls forward, driving the machete deeper into his skull. Tommy and Trish embrance, but Tommy looks down and sees that Jason is still faintly alive. Tommy, in a rage, grabs the machete and begins to hack at Jason repeatedly, screaming "DIE!! DIE!!" as Trish screams out Tommy's name hysterically.
The film cuts to Trish in the hospital, talking to a doctor. Tommy comes in and in a brother-sister moment, Tommy embraces his sister, only for Tommy to look over Trish's shoulder into the camera with an evil stare.