Two months after the events of the first film, Alice Hardy, the sole survivor of the killing spree, is recovering from her trauma. In her apartment, she finds the head of Pamela Voorhees in her refrigerator and is brutally murdered by an unseen assailant with an ice pick to her temple.
Five years later, Paul Holt hosts a camp-counselor training camp at a building near Crystal Lake. Included among the counselor hopefuls are lovers Jeff and Sandra, troublemaker Scott, sporty Terry, wheelchair-bound Mark, sweet-natured Vickie, jokester Ted, and Paul’s assistant, Ginny. At the campfire that night, Paul tells them the legend of Jason Voorhees to scare the other counselors from entering Camp Crystal Lake. That night, local character Crazy Ralph wanders onto the property to warn the kids, but is garroted with barbed wire by an unseen assailant. The next day, Jeff and Sandra go to Camp Crystal Lake and find a recently-killed animal that looks like Terry’s dog, Muffin. They are discovered trespassing by the sheriff and returned to the camp. As he is leaving, the sheriff spots a mysterious figure on the road and chases it into the woods, coming across a run-down shack. As he investigates, he discovers a sight that horrifies him, moments before he is killed by a hammer claw to his skull.
Back at camp, trainer Paul offers the others “one last night on the town”, but makes Jeff and Sandra remain behind as punishment for their earlier excursion. Terry stays behind to look for Muffin, Scott stays behind to flirt with Terry, Mark doesn’t want to go, and Vickie decides to stay with Mark. Terry goes swimming and Scott plays a prank on her by stealing her clothes. He gets caught in a rope trap and while he is hanging upside down from a tree (and Terry goes to find a knife to cut him down), the mysterious killer slits Scott’s throat with a machete. Terry returns to find the hanging body and is then attacked off-screen.
At a noisy bar in the nearby town, Ginny muses aloud that if Jason were still alive, he would have turned into a feral man, having witnessed his mother’s death.
Back at the camp, while Mark is waiting for Vickie to finish getting ready for their lovemaking, he is murdered with a machete to the face and his wheelchair is pushed down the long flight of outdoor stairs. The killer then moves upstairs and impales Jeff and Sandra with a spear as they are having sex. Vickie returns and comes across her friends’ bodies, before she is stabbed by the killer, who is wearing a burlap sack to conceal his face.
Ginny suspects something is wrong when she and Paul return to find the lights out and the place in disarray. The killer creeps through the dark and attacks Paul before turning on Ginny who runs in terror. She is chased through some of the cabins before fleeing into the woods, and eventually comes across the old shack. After barricading herself inside, she finds a rough altar with Pamela Voorhees’s decomposing head on it, surrounded by a pile of the killer’s victims, revealing that Jason Voorhees is the killer. Realizing this, Ginny quickly puts on Pamela’s sweater and tries to convince Jason that she is his mother. The ruse fails when he spots his mother’s head on the altar and attacks Ginny. Paul suddenly intervenes and attacks Jason but is quickly overwhelmed. Just as Jason is about to kill Paul, Ginny picks up the machete and slams it down into the murderer’s shoulder, seemingly killing him.
Paul and Ginny return to the cabin and are greeted at the cabin door by Muffin the dog, alive and well. Just as they feel at ease, Jason bursts through the cabin window behind Ginny and tries to drag her out. She then awakens, being loaded into an ambulance, calling out to Paul, who is nowhere to be seen. Meanwhile, back at the shack in the woods, Pamela’s head remains on its altar.