The film opens with a brief opening title/credits sequence showing a Jack-o-Lantern accompanied by John Carpenter's famous theme music. The shot slowly moves closer and closer. Once it stops, the lighting quickly changes to reveal that the eye and nose form the shape of a man holding a knife.
The film begins on October 31, 1963, a cold Halloween night in Haddonfield, Illinois. In the middle of a street stands a two-story white Victorian house. Someone begins walking toward it, and sees a 17-year-old Judith Margaret Myers (Sandy Johnson) through the front door, kissing her boyfriend (David Kyle) passionately.
As they move towards the couch in the living room, the person outside follows them around the side of the house and watches through the window. The boyfriend asks Judith if they are alone and she replies that someone by the name of Michael is around someplace. Her boyfriend puts on a clown mask jokingly and kisses her. She laughs and tells him to take it off and he suggests that they go upstairs to have sex. They run up the staircase as the person outside the house follows them back around to the front.
The Shape sees the light in the upstairs window go out and begins swiftly walking back to the side of the house through the kitchen door. Quickly, the Shape opens a drawer and grabs a very sharp knife. It begins slowly creeping through the dining and living rooms and reaches the bottom of the staircase. There, Judith's boyfriend is putting his shirt back on and leaving the house. Judith calls down the staircase for him to call her the next day. He takes one last look upward and quietly leaves.
As soon as this happens, the shape begins walking up the stairs into the dark hallway. Looking down, it sees the clown mask on the floor. The Shape picks it up and pulls it over its face. Entering Judith's bedroom, it sees her sitting in front of her mirror, nude except for a pair of panties, brushing her hair. She has her back to the shape and is unaware that it is standing there.
The Shape walks up behind her and she finally feels its presence. Swirling around, she sees that it is Michael (Will Sandin), the person previously referenced, and is annoyed, quickly covering her breasts. But suddenly, Michael raises the knife and begins stabbing her repeatedly in various areas of her body. After a few seconds, she falls to the ground, covered in blood.
Swiftly, the shape walks out of the bedroom, down the stairs, and out the front door into the yard as a car pulls into the driveway. They are Mr. and Mrs. Myers, Judtih's parents. Seeing Michael, they pull off his clown mask, revealing the face of a young 6-year-old boy staring into space with a blank expression, still clutching the knife in one hand.
On October 30, 1978, nearly 15 years later, in Smith's Grove, Illinois, a doctor by the name of Sam Loomis (Donald Pleasance) and his assistant, a nurse named Marion Chambers (Nancy Stephens), are driving to the local sanitarium to have Michael Myers, now 21 years old and still very dangerous, taken to a hearing in front of a judge, where his future incarceration will be determined. In the car is a matchbook with an advertisement for the Rabbit in Red Lounge.
As Marion quietly smokes a cigarette, she questions Dr. Loomis's desire to do this when suddenly, they see that several mental patients are lurking around the entrance to the sanitarium. Dr. Loomis rushes out of the car into the rain to see what is going on just as a patient leaps on top of the car. It is Michael Myers (now played by Nick Castle). Marion rolls down the window to see what he is doing up there and his hand reaches through the window and grabs Marion's hair, causing the car to swerve off the road.
He and Marion struggle for a moment until she pulls away and moves toward the other side of the car. Instantly, Michael's hand appears at the window behind her and shatters it. Marion, startled, opens the car door and jumps out. Michael climbs in and drives away. Dr. Loomis runs over to Marion and consoles her, telling her that the evil is gone.
The next day, on Halloween in Haddonfield, a young girl of about 17 years old named Laurie Strode (Jamie Lee Curtis) is leaving her house to walk to school. On her way out, her father Morgan Strode (Peter Griffith), a real estate agent, tells her not to forget to drop a key off at the old abandoned Myers house because a family is coming by to look at it later. As Laurie continues down the streets, an 8-year-old boy, Tommy Doyle (Brian Andrews), walks up to her. Laurie is babysitting him that night.
As they walk together, the Myers house comes into view. Tommy tells Laurie not to go up there because something bad happened there once. As she drops the key under the mat, someone appears inside and looks through the front door at her. Laurie leaves and continues walking with Tommy, who says he received the information about the house being haunted, from Lonnie Elamb (Brent Le Page).
Laurie says Lonnie will probably never get out of the sixth grade. Tommy leaves soon after this, and Laurie walks on, singing to herself, when suddenly, the Shape from inside the Myers house appears right behind, staring after her.
Meanwhile, back in Smith's Grove, Dr. Loomis is debating with another doctor, Terence Wynn (Robert Phalen), about whether or not Michael has returned to his hometown in Haddonfield. Dr. Wynn doesn't think Michael could drive to Haddonfield and blames Dr. Loomis for not warning the staff about the potential for Michael's outburst. Dr. Loomis angrily gets in his car and drives away.
Later that day, Laurie sits in English class at Haddonfield High School when she sees the Smith's Grove Sanitarium car parked on the street outside the window. Michael stands beside it, in a greyish-white mask, watching her. Laurie is frightened and continues to look back, when suddenly, the teacher calls on her to answer a question, startling her. Laurie responds to the question, and when she looks back through the window, Michael and the car have disappeared.
Meanwhile, at the Haddonfield Elementary School, the bell rings, and children begin running out of the building. Tommy is leaving school carrying a large pumpkin, followed by several bullies who are taunting him about how the boogeyman is coming to get him that night. Tommy is confused, not knowing about the boogeyman, but is also frightened. He begins to run away, but he trips and falls, squashing the pumpkin.
The bullies laugh and run out through the playground gate, when instantly, Michael jumps out and grabs one of them. Terrified, the bully scurries away. Michael climbs back into the Smith's Grove Sanitarium car and begins to drive alongside Tommy, who is walking down the sidewalk sadly.
On the highway somewhere between Smith's Grove and Haddonfield, Dr. Loomis stands at a telephone booth, speaking with someone from the Haddonfield police department, who seems resistant to the idea that Michael could be coming to Haddonfield. Dr. Loomis, frustrated again, hangs up the phone.
But before he drives away, he notices a tow truck parked nearby in a grassy area. Inside, he finds Michael's white hospital gown and the Rabbit in Red Lounge matchbook. Now Dr. Loomis is sure that Michael is in Haddonfield and rushes back to his car, not noticing the unclothed body of the tow truck driver (Barry Bernardi) lying in the nearby grass.
Meanwhile, the students are being released at Haddonfield High School. Laurie is walking home with her friend Lynda van der Klok (P. J. Soles), who's complaining about how many school activities she will have to be involved with in the next day, being such a popular and beautiful girl. Laurie, however, seems to be a quiet bookworm who has nothing to do except babysit.
As they walk along, another young girl named Annie Brackett (Nancy Loomis) finds them. Annie and Laurie talk about their babysitting assignments for that night; Laurie is babysitting Tommy, and Annie is babysitting Lindsey Wallace (Kyle Richards), who lives across the street a few houses away. While they are talking, the Smith's Grove Sanitarium car goes speeding down the street.
Annie yells down the street at the driver that speed kills and, suddenly, the car stops but instantly speeds back up and leaves again. Forgetting this matter and not realizing that they had a close encounter with Michael, Annie and Lynda share a cigarette and begin devising a plan to get Lynda and her boyfriend, Bob, together at the Wallace house where Annie is babysitting, to have sex.
Laurie and Annie continue on, when suddenly, Laurie sees Michael hiding behind a bush ahead of them. Annie doesn't see him, and when she rushes over to the bush, there is nobody there. Annie thinks that Laurie is seeing men behind bushes because she is desperate for a date. Annie reaches home, leaving Laurie behind, who still thinks that there was someone behind that bush.
Looking behind her, she accidentally runs into Mr. Brackett (Charles Cyphers), Annie's father and the sheriff of Haddonfield. Laurie is startled, and Brackett, the local sheriff, replies that it is Halloween and everyone is entitled to one good scare. Laurie laughs and walks back towards her house.
Walking into her bedroom, she looks out of the window and sees Michael standing in her neighbor's back yard, staring up at her from between two rows of clothes hanging on a line. One second he is there, the next second he is gone. Suddenly, the phone rings, and Laurie runs over to it. When no one answers and she only hears the sound of chewing, she hangs up, terrified. The phone rings once again and it is Annie on the other end.