Seven sorority sisters, Katey (Katherine McNeil) Morgan (Jodi Draigie), Stevie (Ellen Dorsher), Liz (Janis Zedo), Vicki (Eileen Davidson), Jeanie (Robin Meloy), and Diane (Harley Jane Kozak) pull a prank on their strict house mother, Mrs. Slater, who is known for carrying a sharp walking cane. Their plan is to put Mrs. Slater's cane out in their dirty pool and force her at gunpoint to swim through and retrieve it. The prank goes awry when Vicki, who was holding the gun, accidentally shoots Mrs. Slater. The girls all agree to hide Slater's body in their dirty swimming pool until a graduation party being held in the house is finished. As the party begins, a guest wanders around the pool and an unknown figure stabs him through the neck with Mrs. Slater's cane and drags him away.
When other guests attempt to throw Jeanie in the pool, they are stopped by the sorority sisters. Later, to prevent the pool lights from coming on revealing Slater's body, Stevie is forced to go into the power room of the house located in the basement. While doing so, she is brutally stabbed to death by Mrs. Slater's cane. Katey meets her date for the party, Peter, but she and the other girls worry why Stevie has not returned from the power room yet.
Vicki orders the girls to search around the house for Mrs. Slater. Morgan enters Mrs. Slater's room and begins to sort her clothing when Mrs. Slater's body falls on her. She is joined by the sisters who wonder how her body was moved to the bedroom. Morgan subsequently dies when she gets lured to her bedroom door and stabbed through the stomach by the cane. Having found Slater's body, the sisters decide to bury it in a nearby graveyard. Meanwhile Katey goes into the attic and discovers children's toys, a clown statue, and a dead bloodied bird laying in its cage. Diane is waiting in her van for the other girls, but the figure stabs her to death with the cane from the sunroof. Jeanie, the most nervous member of the group, is also attacked while running back into the house but manages to escape. After informing Katey about the attack, Jeanie is chased into the upstairs bathroom. The killer finds her and enters the stall, decapitating her with the butcher knife.
The party goers finally depart, leaving Katey and Peter in the house. After realizing that Jeannie and the others are missing, Katey tells Peter to go home and finds a number on a chain she found near the gate. The chain has the phone number for a Dr. Beck who soon arrives and asks Katey where Slater is. The two discover the bodies of Stevie, Morgan, and Diane in the pool. Meanwhile Vicki and Liz attempt to bury the body in the graveyard. Liz brings the van around and gets her throat sliced by the cane's razor sharp handle. Vicki calls for Liz and when she gets no answer, she approaches the van and discovers Liz's body. Vicki turns around, only to be hacked to death with the cane by the assailant. Katey and Dr. Beck arrive at the graveyardonly to discover their bodies.
After giving her a sedative, Dr. Beck reveals to Katey that Mrs. Slater had a son named Eric who was horribly deformed and mentally underdeveloped. Eric had actually been living in theSorority House's attic and witnessed the shooting death of his mother. He is now exacting revenge on the girls for their irresponsibility with the prank. Katey is used as bait to lure Eric out of the attic so Dr. Beck can shoot him with a tranquilizer gun. However, the plan backfires when Peter comes through the door and is shot. Dr. Beck is hacked twice in the stomach by Eric who also throws him down a flight of stairs while Katey grabs the hand gun that was used on Mrs. Slater and finds Dr. Beck's body. She flees to the upstairs bathroom and discovers Jeanie's severed head in the toilet, staring back up at her. A traumatized Katey climbs up into the attic and is attacked by Eric. As Eric approaches her, Katey seemingly kills Eric by stabbing him multiple times with a pin sticking out of a doll's neck, and he falls through the attic hatch to the floor below. Katey peers down and, believing that he is dead, she becomes relieved. However, Eric was only incapacitated, as he opens his eyes before the film ends.
- Kate McNeil as Katherine 'Katey' Rose
- Eileen Davidson as Vicki
- Janis Zido as Liz
- Robin Meloy as Jeanie
- Harley Jane Kozak as Diane
- Jodi Draigie as Morgan
- Ellen Dorsher as Stevie
- Lois Kelso Hunt as Mrs. Dorothy Slater
- Christopher Lawrence as Dr. Nelson Beck
- Michael Kuhn as Peter
- Michael Sergio as Rick
- Charles Serio as Eric