The film begins on Valentines Day, 1984, in a fraternity house. Six brothers, Dave, Bruce, Paul, Jim, Nick and John, plan to play a prank on awkward pledge Mervin Prescott, who has a massive crush on Lilly, a girl from the neighboring sorority. They trick Mervin into going to an old schoolhouse were a child murderer supposedly hid, so that he can have sex with Lilly, but Bruce jumps out wearing a clown costume, (Mervin is terrified of clowns). Mervin, however, is so horrified that he falls backwards out of a window and onto an old metal fence and is imapled to death. Both the fraternity and the sorority cover up the death as purely accidental, and those responsible soon resume their normal lives.
One year later, a party is thrown at the fraternity house to celebrate Valentine's Day. Frat brother Andy and sister Janie (who was involved in the prank) sneak upstairs during the party to have sex, but someone sneaks in through the window behind them and stabs Janie to death with a metal crucifix, and then decapitates Andy.
Later, John goes to the bathroom, but the killer is hiding behind the shower curtain, and stabs him with a garden shears. They then sneak into his room and retrieve the clown costume Bruce used in the prank.
The group involved with the prank, Lilly, Val, Jenna, Alice, Bruce, Dave, Paul, Nick and Jim head out to the place were Mervin died to "celebrate his life." Lilly admits to Bruce that she feels guilty about the prank and Bruce, taking advantage, seduces her and sleeps wih her in the van. Meanwhile, John's girlfriend Meg follows the group to tell them she couldn't find him (they sent her to find him) but the killer follows her and then hits her with a hatchet.
At the schoolhouse, Alice and Nick decide to scare Bruce and Lilly in the van, but Bruce has already told Lilly to expect it and pay no attention. The killer then attacks Nick and stabs him to death and when Alice tries to alert Bruce, she to is killed and Bruce ignores her. The killer then sets the van alight, killing Bruce and Lilly.
Meanwhile, at the party, two new sisters, Beth and Stephanie, begin to wonder where the group has gone and catch word of the incident from their friend Sandy. They realise that they must be at the schoolhouse, and set off.
Val and Paul discoverws the bodies of Alice and Meg and are attacked by the killer, when Paul is decapitated in the fight. Val gets back into the schoolhouse and warns the others, and they see to killer outside and realised there is no escape. Jim decides to try and run to the van and start it up (they have no idea it was torched and simply presume Lilly and Bruce to be dead) However, when he reaches the van, he is horrified and then stabbed in the back with a machete. At the party, Sandy realises what happened to Stephanie and beth and goes to alert the house mother. Meanwhile, Val goes mad and tries to run back to the party but is killed and Jenna is wounded while trying to save her.
From a window, the group witness Beth and Stephanie arriving and Stephanie's subsuquent murder. Dave runs outside to try and save beth, but he has his head smashed with a shovel while Beth escapes into a nearby field. Jenna chases the killer and fights with them, but the killer then reveals that they are carrying a sander and shoves into her mouth. Beth returns with a pickaxe and strikes the killer through the head, killing them. Before she can remove the mask, Sandy and the house mother arrive, who is carrying a gun. sandy is relieved to see that Beth is safe, but is suddenly shot dead by the house mother. The mother relieves that she was actually Mervin's mother, but she gave him over to the father, who as married, and paid for him to attend the fraternity so that she could watch over him. When he died, his stepmother drank herself to death and his father wanted revenge, so he killed everyone releated to the prank and anyone who got in the way. She had disclosed the names to him and plotted with him for revenge. She then tells Beth that she'll have to kill Beth to keep her quiet. Just then, a wounded Sandy smashes her over the head with a large brick, knocking her to the ground. Beth retrieves the gun as the house mother finishes Sandy off with the brick. She stands and asks Beth if she's really going to kill, but Beth refuses to. The house mother then lunges forward, but Beth fires the gun and kills her. As Beth collapses onto the ground in exhaustion, the killer's eyes are shown opening in the mask.