A day after the events of Friday the 13th Part 3, police and paramedics are busy cleaning up the mess deformed serial murderer Jason Voorhees left at Higgins Haven, including the defeated hockey-masked killer himself. Once delivered to the Wessex County morgue, it turns out that Jason is still alive. He rises, kills morgue doctor, Axel, and Nurse Morgan, then makes his way back to Crystal Lake. A group of friends (Paul, Sam, Sara, Doug, Ted, and Jimmy) have rented a house on Crystal Lake. On the way there, the group passes Mrs. Voorhees' tombstone and a female hitchhiker, who becomes Jason's next victim, stabbed in throat while eating a banana. Next to the rental house is the Jarvis home. The group meet Trish and Tommy when they arrive. The next day the group befriends twins Tina and Terri, who live in the area, and they all go skinny dipping at Crystal Point. Trish and Tommy, driving by, stop to see who's at Crystal Point and the group invites Trish to a party that night. Trish's car breaks down a bit further along the road, and they are helped by Rob, a hiker with mysterious reasons for visiting Crystal Lake, who soon becomes good friends with Trish and Tommy.

Next door, the kids are enjoying themselves listening to music. With four girls and four boys, each now has a date. However conflict ensues as some of the kids switch dates. These conflicts prove to be the least of their troubles as Jason stalks and kills them one by one. Sam is impaled from under a raft while skinny-dipping. When Paul goes out to be with her, he has a speargun rammed in his groin. Terri decides to leave early and is about to get on her bike but has a spear rammed into her back. Jimmy decides to celebrate scoring with Tina with a bottle of wine. While searching for a corkscrew, Jason appears from the shadows and slams it on Jimmy's hand and hits him in the face with a meatcleaver. Meanwhile Tina looks out the window and is grabbed and thrown two stories down landing on the car. While Ted watches vintage pornography he gets too close to the projector screen and is stabbed in the head by Jason who plunges a kitchen knife though the screen from the other side. After Doug and Sara finish making love in the shower, Jason attacks Doug, crashing his head against the shower tile. He then kills Sara by throwing an axe through the front door when she tries to escape.

At the Jarvis house, Trish and Tommy find their mother missing, so Trish goes to Rob for help. Rob explains that he's looking to get revenge for the death of his sister, Sandra Dyer (killed by Jason in Part II). Trish and Rob take Gordon (the Jarvis family dog) next door to see what's going on. Tommy is left at home, and finds Rob's newspaper articles about Jason. At the house, a frighted Gordon jumps though a second story window apparently killing himself. Jason kills Rob, and Trish flees back to her home intending to warn Tommy. Tommy shaves his head and makes himself up to look like Jason, which is effective in distracting Jason long enough for Trish to be able to attack Jason with his machete. She just misses him, but manages to knock the hockey mask off his face. While Jason heads for her, she drops the machete to the floor. Tommy picks it up and swings it at Jason's head. Jason then falls to the floor, causing the machete to dig further into his head. As he embraces his sister, Tommy sees Jason begin to move, loses control and, hacks Jason repeatedly with the machete, while Trish screams his name.