On March 12, 1945 in Avalon, California, during World War II, a woman named Rosemary writes a letter to her boyfriend, breaking up with him. Three months later, Rosemary is attending a graduation dance with her new boyfriend Roy, who suggests they go out to lovers lane. While there, they are attacked by a mysterious prowler in an army combat uniform, who impales them both with a pitchfork and leaves behind a rose.
35 years later, college senior Pam MacDonald is making last-minute arrangements for that night’s graduation ball, the first to be held since the 1945 murders. Helping Pam are her friends Lisa, Sherry and Sherry’s love interest Carl. Later while visiting her boyfriend and the town’s deputy, Mark London, Pam overhears a report of a prowler. She expresses her concern about Mark’s safety because the sheriff is leaving town for a fishing trip. Mark reassures her and promises to meet her at the dance. Pam then heads back to the dorm to get ready and eventually leaves with her friends. Sherry stays behind to shower and receives a surprise visit from Carl. While undressing, he is attacked and killed with a bayonet. The killer then impales Sherry with a pitchfork in the shower.
At the dance, Pam spots Mark and motions him over. As he walks towards her, Lisa asks him to dance. Mark agrees, which upsets Pam. Mark then walks over to Pam and the two share a brief but tense exchange. A moment later, Lisa walks up and bumps Mark, which causes him to spill his drink on her dress. She returns to the dorm to change and is chased by the same prowler, but escapes. She runs outside into the wheelchair-bound Major Chatham who grabs her arm. Pam escapes his grasp and soon reunites with Mark. She tells him about the prowler so he investigates. After checking the dorm, Mark only finds bootprints and wheelchair tracks outside. The two then go investigate the Major’s home. Pam realizes that his daughter was Rosemary and that her killer was never caught. Convinced the prowler from earlier is the same killer, Mark and Pam head back to the dance and warn the chaperone, Allison, about the possible danger.
Meanwhile, Lisa goes out to a nearby pool to cool off and encounters the killer, who slits her throat. Paul, Lisa’s boyfriend, is arrested by Mark for public intoxication. Allison goes to find Lisa but is stabbed and killed also. Mark and Pam go to investigate the cemetery and discover an opened grave with Lisa’s body in it. Mark tries to call the cabin that the sheriff went to, but is ignored by the site worker. Mark next calls the state police for help. He informs Pam that the state police told him that the reported prowler had been caught three hours earlier, and could not have killed Lisa.
Pam suspects that Lisa’s killer is the same killer who murdered Rosemary and her boyfriend in 1945. They go to investigate Major Chatham’s house again. Mark is attacked as the prowler chases Pam through the house. Otto appears and shoots the prowler, but he recovers and shoots Otto dead with a shotgun before again attacking Pam. While pinned to the ground, Pam manages to unmask the prowler, revealed to be Sheriff Fraser. Pam wrestles with the shotgun from his hands, then eventually puts it under his chin and pulls the trigger, killing Fraser.
The next day, Mark returns Pam to her dorm and she goes up alone. Discovering Sherry and Carl’s bodies in the shower, she screams as Carl seems to come to life. She realizes Carl is dead and that him grabbing at her was a hallucination.