The day following the chaotic events of Friday the 13th Part III, law enforcement and emergency responders are engrossed in the grim task of clearing up the aftermath left behind by the deformed mass murderer, Jason Voorhees, at Higgins Haven. As they transport Jason's lifeless body to the Wessex County morgue, an unexpected twist occurs—Jason has survived! He revives, swiftly dispatching morgue doctor Axel with a brutal hacksaw attack that snaps his neck, and subsequently slays Nurse Morgan by stabbing her in the stomach before making his way back to Crystal Lake.
Meanwhile, a group of friends including Paul](/actor/alan-hayes), Samantha, Sara, Doug, Ted, and Jimmy have rented a delightful getaway house by Crystal Lake. Their journey takes a dark turn as they pass by Mrs. Voorhees' grave, and during a pit stop, they unknowingly allow a female hitchhiker (Bonnie Hellman) to become Jason's next target, as she meets her end with a knife to the throat while indulging in a banana.
Upon their arrival at the rental property, the group encounters 17-year-old Trish Jarvis](/actor/kimberly-beck) and her 11-year-old brother Tommy. The evening unfolds with fun and laughter, yet remains uneventful. Trish is eager to join the group, but her mother (oan Freeman) disallows her from doing so. Meanwhile, Tommy, a young video game junkie, is occupied in his room creating latex masks and engrossed in his Atari games.
The next morning, the group meets twin sisters Tina and Terri, who have charming British accents, and the group heads to Crystal Point for some skinny-dipping. Trish and Tommy, while passing by, are invited to a party that night. However, their plans take a detour when Trish's car breaks down, leading to help from [Rob, a hiker with mysterious intentions who quickly becomes friends with Trish and Tommy.
As night falls, the kids entertain themselves with music and lively dancing, forming pairs and enjoying each other’s company. However, tension arises when date-swapping occurs amongst the group. Just when they think the turmoil is subsiding, Jason begins his murderous spree, stalking and eliminating them one by one. Samantha attempts a daring swim and is brutally impaled from beneath the water by Jason. In a tragic moment, Paul discovers her lifeless body but meets his own demise when stabbed in the groin by a spear.
When rain begins to pour, Terri makes a hasty exit but is pierced through the back with a spear. After a passionate encounter with Tina, Jimmy searches for a corkscrew only to have Jason pierce his hand before delivering a fatal blow with a meat cleaver to his face. Upstairs, Tina looks outside only to be seized and thrown from a window, landing harshly on a car below.
Meanwhile, a distracted Ted finds himself on the receiving end of a kitchen knife through a projection screen while watching vintage films. After Doug and Sara finish in the shower, Jason viciously attacks Doug, smashing his head against the tiles, before killing Sara with an axe when she tries to escape via the door.
At the Jarvis residence, Trish and Tommy realize their mother is missing. In desperation, Trish seeks help from Rob, who reveals his quest for revenge against Jason for the death of his sister, Sandra Dyer. They take Gordon, the family dog, to investigate next door, but the brave pup panics and escapes through a window. Tragically, Rob falls victim to Jason as well, prompting Trish to rush home to warn Tommy.
In a harrowing chase through the houses, Tommy shaves his head and creates a mask resembling Jason's, momentarily distracting the killer. This gives Trish a chance to attack him with a machete, narrowly missing but dislodging Jason's iconic hockey mask. Overwhelmed by Jason's appearance, she drops the machete in horror. However, Tommy regains control, seizing the machete and inflicting multiple blows onto Jason, who ultimately collapses.
In the closing scene, a weary Tommy visits Trish in the hospital, and they embrace, sharing a moment of relief as they believe their nightmare is over. Yet, the film concludes with an unsettling shot of Tommy gazing psychotically at the camera.
