A group of teenage friends are playing volleyball at a Los Angeles beach. Jackie is sitting with Diane talking when a strange man appears and sits at the beach near them and stairs at the girls. Duncan walks up to the weirdo and tells him to keep his eyes to himself. Duncan then mentions to Diana and Jackie about a party, but they tell him that it is a slumber party and guys are not invited. Juliette goes to get the volleyball and talks to her boyfriend Ken. As they are leaving, Jackie goes back to for something and drops her address book. When Sarah gets into her car, she is grabbed by an unseen person and a drill is run through her stomach from behind the car seat.
Frank drives Jackie home and they kiss for a minute. When they approach the front door, they see that it is wide open. Inside the house is Morgan, who though it was an open house since Jackie's parents are moving. Jackie says that he can stay and look around for a while. After Jackie listens to a mysterious voice on her answering machine, an apparently nervous Morgan finally leaves.
Later, Jackie's friends arrive for the slumber party and telling her that Sarah did not answer at home and suggests that she may have flaked out on them. Morgan calls the girls (while he is spying on them through a telescope from his house across the street) and he asks Jackie if he can look at the house again. Jackie responds negatively. A little later, when Juliette and Maria playfully perform a striptease, Frank, Tom, and Michael scare them with masks. Jackie tells the guys to leave.
The weirdo from the beach is hanging outside the house, spying on the girls. Frank and Tom go to get something for the girls, and Michael goes to apologize for scaring them. He knocks on the front door, and girls ignore him. When he walks away he has an encounter with a masked person and he runs back to the door and bangs for help. He gets impaled with a "house for sale" sign post. The masked killer drags his body away and returns the post to the front yard.
A little later, Ken calls Juliette wanting to speak with her, and she invites him over. Duncan waits outside and sees a pizza delivery girl arriving with a pizza. He bribes the pizza girl to switch shirts and let him have the pizza. Duncan delivers the pizza to the girls and to say "I'm sorry" to them for the prank earlier. The girls let him into the house. The pizza girl (wearing Duncan's shirt) is walking down the street when she is chased by the unseen killer and drilled in the back.
At the house, Jackie is talking with Diane when they hear a noise and see the weirdo outside. Jackie calls the police, but Officer O'Reilly on the other end of the line thinks that it's "just a bunch of girls with overactive imaginations." The girls let Frank and Tom inside, and Ken appears right behind them and enters the house too. Frank gives Jackie some flowers and apologizes. Meanwhile, the weirdo from the beach gets into the basement.
Susie shows her boyfriend, Tom, a swordfish on display in the basement and the two of them kiss. Upstairs, Juliette is upstairs making out with Ken and when her hand moves over to his crotch, he stops her. Janie hears them and tells the others "they're doing it!" Tom accidentally drops the pizza on the carpet and Susie gets some bleach to clean it up. Juliette goes into the bathroom to shower, and Ken leaves. She finds a vibrator in the bathroom, she plugs it in and laughs. While Juliette is in the bathtub, the lights go out and someone enters the bathroom, turns on the vibrator, and throws it into the bath water, electrocuting Juliette.
A few minutes later, Maria enters the bathroom, dries up some of the water that is on the floor, opens up a closet to get another towel and finds the dead Juliette. The group gathers together in the living room and Maria suggests that the weirdo from the beach killed their friend. Jackie calls the police again, but Officer O'Reilly still does not take their call seriously and hangs up. Ken suggests that he make a run for it and find his uncle, who used to be a police officer. As he runs out the front door, Tom joins him. As the two are running down the street, Ken stops by a lumberyard and suggests getting tools for better weapons. Tom finds a sledgehammer and gives it to Ken, who suddenly whacks Tom with it. Tom and Ken fight until Ken finally gets a chain saw and slices into Tom's legs. After Ken leaves, Tom sits up and begins to crawl away.
Back at the house, Jackie goes with Frank to check the basement. After seeing the swordfish with the "sword" broken off, the find the missing piece sticking in the dead weirdo in a trunk. They realize that the weirdo is not the killer. Just then Ken calls the house from a payphone where he tells them that his uncle said it would be safer to stay inside the house. After Ken hangs up, he goes to a van and inside are photos of his cop uncle, the dead bodies... and the drill. Ken is the driller killer.
Back at the house, Duncan goes to answer a knock on the front door and Ken barges in and drills him in the chest. Frank jumps on Ken from behind and the two men struggle. Morgan (still looking through his telescope) calls the police and reports a disturbance. Jackie hits Ken with a lamp, and Frank is knocked down by the drill. Jackie sits by Frank and tells Ken that he killed him. The other girls run but cannot get the back door open (because Ken had apparently lodged something between the sliding doors). Maria is separated and Ken approaches her. She sees Tom dead by the window after he bled to death after crawling back to the house. Janine runs in and knocks Ken on the head with a glass bowl. Janine and Maria run to the door, but Maria is swiped by Ken's drill. Janine jumps through the glass back door and collapses on the patio. "What a waste," Ken says to himself as he starts up the drill to finish Janine off.
Ken goes upstairs to look for the remaining girls. He walks into a room where the wounded Maria and terrified Susie are hiding. He finds Susie in a closet, pulls her out, and throws her onto a bed where he hits her. After Maria comes out of hiding, she hits Ken on the head with a lamp. Maria, Jackie, and Diane run down into the basement. They try crawling out of a window to escape, but Ken appears. Jackie shoots him with a spear gun, hitting him in the leg. They run back upstairs and find the dead Janine and her innards on the patio. When the wounded and angry Ken staggers up from the basement, Susie pours liquid bleach in a bucket and throws it at his face, blinding him. He swings wildly with his drill and hits Maria again, who pleads with Ken not to kill her. She says she knows he has been hurt and she seems to relax him by letting him touch her. He blindly gets on top of her, and Maria reaches for the drill. Ken becomes upset when her other hand moves around too much and correctly assuming she's trying to get at the drill, he manages to get to it first and kills Maria.
Still blinded, Ken hears Susie's voice coming from the kitchen. Jackie and Diane are standing by the front door to the kitchen waiting for him to make his move. Ken heads for Susie until Diane calls out and gets his attention. They knock the drill away from Ken and throw a volleyball net over him, while Diane whacks him with a wooden baseball bat until he is knocked out. Having finally subdued him, Jackie goes to call the police in the next room. Ken wakes up and his vision begins to return as Susie and Diane stand guard over him. Ken thinks back to his insane Uncle Billy whom apparently motivated him to become the killer that he is. Ken then jumps up and grabs Diane. Susie tries to stab him, but he knocks her to the side and fatally stabs Diane. Jackie runs in, seizes the drill, and drills Ken to a bloody death. Relieved that it's finally over, Jackie finds on the dead Ken a photograph of Uncle Billy with a young Ken.