In a suburban Los Angeles neighborhood, a paperboy tosses a newspaper with the headline that screams: "MASS MURDERER OF 5 RUSS THORN ESCAPES". In one house, 18-year-old Trish Deveraux (Michele Michaels) wakes up, dresses, and gathers some dolls to throw away. Over her radio alarm clock, a news reporter talks about a mass murder, named Russ Thorn, who escaped from the state insane asylum where he was incarcerated since 1969 for killing five people in nearby Venice, California. Outside, Trish's parents are leaving for the weekend and her mother tells her that their next door neighbor Mr. Contant (Ryan Kennedy) is there if she needs help with anything. Her parents leave and Trish throws her bag of old dolls in a trash can and begins to walk to school. As she is leaving, someone takes one of the dolls out of the trash.At the local high school, two guys, named Jeff (David Millbern) and Neil (Joe Johnson), are talking about girls and about rumors of a slumber party that Trish is throwing at her house tonight. Jeff hits on a telephone repairwoman, named Mary, as she is fixing a phone line to one of the buildings on the school grounds. Neil pulls Jeff aside and the boys keep walking as the woman stands by her van. Suddenly, a man pulls Mary into the van and drills her to death with a large power drill.In the school gymnasium, Trish is playing basketball with the rest of her teammates. Trish is part of the school's girls basketball team. Jeff and Neil arrive to watch the girls practice and discuss the new girl, Valerie Bates (Robin Stille) whom is also on the court. The girls coach Jana (Pamela Roylance) yells to get the girls motivated. Valerie impresses the coach by scoring a few baskets, but the others ignore her because she is the new girl at school. In the locker room showers, Valerie continues to be ignored until Trish approaches her and compliments her on her basketball skills. A little later, Trish suggests to her teammates Diane (Gina Mari), Jackie (Andree Honore) and Kim (Debra Deliso) about inviting Valerie to their slumber party. But Diane rejects the idea and begins to bad-mouth Valerie to the rest of the girls. Valerie hears the girls and begins to leave until she is stopped by Trish. Valerie coldly says that she cannot come over to Trish's house for the party and she leaves.Outside the gym, the group is leaving as Trish, Jackie, Kim, Diane, and another teammate named Linda (Brinke Stevens) discuss the party and they are joined by Jeff and Neil. They are observed by the killer Russ Thorn (Michael Villella), a wild-eyed, casually dressed, middle-aged man watching them from the telephone repairwoman's van. Linda returns to the gymnasium to her locker to get a book as Russ Thorn watches the group leave, passing by a dumpster, with the dead telephone repairwoman in it, as well as Coach Jana also leaving the gymnasium. As Linda retrieves a book from her locker, she finds that she is locked in the building. As she looks for a way out, Russ Thorn appears and walks up to her in the hallway and attacks her with the large phallic power drill, drilling through her left arm. Linda runs and hides in the dark shower room and under a counter door. After looking around, Thorn sees her blood seeping under the door and drills right through the door. Linda screams, but her screams do not last long. Thorn runs back to the van and drives off.While walking home from school, Diane meets her boyfriend John Minor (Jim Boyce) who surprises her. John asks Diane if she can get around the no-boys-allowed rule at Trish's party. Diane tells John to call her later at Trish's house to ask again.Across town, Coach Jana arrives home only to be surprised when a drill bit crashes through her front door, but it is only her carpenter Pam drilling a peak-hole for her front door. Jana asks the carpenter if she had seen her pet cat, but she hadn't.Meanwhile, Trish arrives home and talks on the phone with Diane until she hears a noise, and is scared by Mr. Contant, who offers to sit and wait with her until the girls arrive.At Coach Jana's house, she hears a noise and opens up a closet door to find her cat, Muffin, jump out and scare her.At nightfall, the driller killer, Russ Thorn, arrives in the telephone repairwoman's van on Trish's street, where he gets out, and lurks in the bushes around Trish's house. Jackie and Kim arrive at Trish's house and show her a bag of marijuana they brought with them and begin to smoke some of it. Mr. Contant walks in and does not seem to mine and he leaves the house, walking next door to the right of Trish's house.Coincidently, the other next door house to the left of Trish's is the Bates home where Valerie is with her younger 14-year-old sister, Courtney (Jennifer Myers), and looking after her while their divorced mother is away for the weekend as well. Valerie checks outside when she hears a noise to see an overturned trash can.Meanwhile, Jeff and Neil arrive and lurk outside the house on the other side to watch the girls change their clothes, smoke more pot, and make a fire in the fireplace. Inside the house, the girls hear a noise and investigate the kitchen, only to find that Diane has arrived. Trish suggests ordering a pizza, and Diane goes outside to get more wood for the fireplace. She meets Mr. Contant, who is chopping snails with a meat cleaver. Diane says goodnight to him and leaves. Just as Mr. Contant is preparing to snag another snail, the driller killer walks up to him from behind and drills him through the neck.A little later, Russ Thorn continues lurking outside the house and just when he is about climb in through a window, Trish appears and closes the window, unaware that he is there. When she looks again, the sees a melted doll (the same one she threw in the garbage earlier), stuck up to the window with the meat cleaver. Trish and Diane go to the garage to check if there is someone there and see that it is not locked. Diane calls John on the phone, while the stoned and giggly girls listen in on the line. Diane hears them and tells John to come over soon. Just then, the lights in the house go out. The girls go to the garage to check the fuse box and find some fuses missing. The are scared by Jeff and Neil, and in turn Jeff gets punched by Trish for scaring them. Trish forces them to re-install the fuses, making the lights in the house come on again and all of them go inside.Next door, Valerie hears a noise again and she sees both trash cans lying down on the ground. Courtney thinks that it just may be some stray dogs looking for food. Courtney looks through her bedroom window at Trish's house, curious about the party going on and she sees John pulling up on the driveway in his car. John honks his car horn and Diane comes out saying she cannot leave right now. John pulls his car into the garage.Meanwhile, Courtney suggests crashing Trish's party while Valerie brushes her hair. Valerie tells her sister that she just wants to be alone and does not feel very sociable. In Trish's garage, Diane and John are making out in his car while the others are inside. John suggests going over to his house for some intimate time and Diane goes in the house and tells Trish that she and John are going out for beer. Trish, aware that Diane only wants to go someplace to have sex with John, replies "you don't have to ask my permission, Diane". Diane returns to the car where she gets in the front seat only to find John's head falling off his body. She screams, but her screams are not heard because the girls inside are running a blender to make margaritas. Diane crawls out of the car and is killed by Thorn, who is still in the garage.Next door, Courtney thinks that she hears honking and screaming, but Valerie says that it's nothing. Courtney again asks Valerie if they could go over to the party, but Valerie again refuses. Courtney retreats to her bedroom while Valerie stays downstairs and turns on the living room TV to watch a movie.At Trish's house, the front doorbell rings and the group thinks its the pizza guy. Jeff asks through the door "what's the damage?" he hears a voice through the door saying "six... so far". Kim and Jackie are on the phone with Coach Jana to ask about an upcoming basketball game, and the front door is opened to find a dead pizza delivery guy, his eyes gorged out, who falls through the doorway and lands on the floor..... with the pizza still in his dead arms. The girls scream and Trish hangs up the phone to call the police, but Thorn cuts the phone lines. Coach Jana hears the screaming on the other line before she gets cut off. Jana tries calling the girls back, but there is no answer. Coach Jana calls Valerie suggesting checking out the house and she says that she will drive over there to help check it out herself.In Trish's house, Jeff and Neil arm themselves with kitchen knives and begin to lock all the doors and windows of the house. After a while, Jeff suggests that they make a run for it to get help. Neil decides to make a run for Valerie's house while Jeff will make a run for Mr. Contant's house to get to a phone. Trish and the girls reluctantly agree. Jeff goes out the back door while Neil runs out the front door. But by splitting up, the guys guarantee their own demise. Jeff has to run through the back yard and through the garage to get out the back way where he finds a dead Diane in the garage. Thorn is in the garage waiting for him and drills Jeff in the back. Thorn then runs after Neil who makes it to Valerie's front door and begins frantically knocking and screaming for help. But Valerie either ignores the knocking or doesn't hear it due to the loud television. Courtney doesn't hear Neil calling for help either because she is on her phone in her bedroom talking to a friend while playing loud rock music on her radio. Thorn approaches and attacks Neil in which he manages to twist the knife Neil has from his hands and gorily stabs the boy to death. When Valerie finally goes to the front door, she cannot see anything as Thorn is carrying Neil's dead body back to the garage and throws him in the trunk of John's car where the dead bodies of John, Mr. Contant, and Diane are stuffed in. Thorn counts the bodies and realizes that one is missing. It is Jeff, who despite being gravely wounded, is crawling back towards the back door.Inside, Trish, Diane, and Kim sit in a circle with knives in their hands. Jackie goes for the dead pizza boy's delivery and enjoys a slice of pizza. The girls hear the wounded Jeff scraching on the back door. But they are too afraid to open the door thinking that it might be a trick. Then they hear Thorn approach Jeff and drill him to death. Meanwhile, Coach Jana gets in her car and begins to drive over to Trish's house, while Valerie goes upstairs to find Courtney gone. Valerie sees her sister walking to Trish's house and runs after Courtney.Courtney sees Valerie approaching and ducks away from her by running around the side of the house. Valerie goes up to the front door and rings the doorbell and knocks on the front door. When she gets no reply, she leaves. Jackie runs to the front door from the kitchen to let Valerie in and when she opens it, Thorn is now standing there and swipes the drill across Jackie's neck, killing her. Trish and Kim run upstairs and barricade themselves in Trish's bedroom.Meanwhile, Valerie finds Courtney in the back of the house and she tells her that she does not think anyone is home after looking in the windows and seeing no one. Valerie tells her that she will check the front door again. She walks inside the house through the front door after finding it conspicuously wide open after knowing that the front door was closed and locked just a few minutes ago. Valerie calls out the girls' names, and, upstairs, Kim wants to respond, but Trish suggests that Valerie might be helping the killer. Looking around the downstairs and finding nobody in sight, Valerie walks out through the kitchen into the backyard to find Courtney gone.Upstairs in Trish's bedroom, Trish and Kim talk about what they are going to do, when Thorn sneaks in through a window and attacks them. Trish defends herself by knocking the driller killer down with a baseball bat and then tries moving the furniture away from the blocked door. But Thorn quickly revives and Kim attacks him with a knife. Thorn twists the knife out of Kim's hands and stabs her to death. Trish runs and hides in her parents' bedroom closet.While Thorn searches for Trish upstairs, Valerie goes outside and finds Courtney and they go back inside Trish's house where they find Kim's dead body in the refrigerator. (Note: there is a minor plot hole here as well as some cheating as one would wonder why in the world would the killer drag Kim, after having killed her upstairs, downstairs to the kitchen to stuff her into the refrigerator, and in such a short time). Both girls scream, and Thorn hears them. Valerie sees the shadow of Thorn on the stairway and hides in the basement while Courtney ducks under the living room couch. Thorn walks downstairs and looks around and sees no one. He closes the front door and drags the dead pizza guy away into the basement, while Valerie hides under the basement stairs. Then, for some odd reason, Thorn lies down on the living room floor and covers himself with a blanket.A few minutes later, Coach Jana arrives at the house and calls out the girls' names and walks inside. Courtney, still hiding under the living room couch just feet away from Thorn, Trish, still hiding upstairs, and Valerie, still hiding in the basement, do not call out in fear of being discovered by the killer. When Coach Jana approaches a figure, covered, on the living room floor, Thorn jumps up and goes after the coach with the drill while she defends herself with a fireplace poker. Courtney intervenes and trips Thorn, who gets beaten by the coach with the fireplace poker, and Trish runs in and stabs Thorn once in the back. He sits back up and slashes Coach Jana across the chest with the drill, killing her, and knocks Trish to the floor. Speaking for the first time, Thorn makes a remark to Trish how pretty she is and that all of her friends are very pretty. The driller killer tells Trish that "it takes a lot of love for a person to do this."Just then, Valerie runs up from the basement, wielding a large machete and swings it at Thorn who runs out the back door and stands by the backyard swimming pool while Valerie continues swinging the machete at him. With a powerful swing, Valerie chops off his drill bit and further attacks, chopping off Thorn's left hand and slashes him in the abdomen in which he falls into the pool. Valerie drops the machete and she and her sister embrace. Thorn crawls out of the pool and jumps atop Valerie to get at the machete. Courtney tries to stop him, but Thorn punches her in the face and knocks her aside. Trish runs out and attacks him again. Thorn knocks Trish down and jumps for Valerie, who grabs the machete, and he lands right on top of it. Thorn falls to the ground, dead at last. The shaken Trish, Valerie, and Courtney look on as police sirens are heard in the distance.