On the night of October 31, 1963, in Haddonfield, Illinois, 6-year-old Michael Myers (Will Sandin) murders his older sister Judith (Sandy Johnson) by stabbing her with a kitchen knife. Fifteen years later, on October 30, 1978, Michael escapes Warren County Smith’s Grove Sanitarium, where he had been committed since the murder. He steals the car that was to take him to a court hearing, the intention of which was for him to never be released.
The following day, Halloween, 21-year-old Michael (Nick Castle), now dressed in a blue jumpsuit and a white mask, returns to his hometown of Haddonfield, and begins stalking high school student Laurie Strode (Jamie Lee Curtis). Laurie informs her friends, Annie Brackett (Nancy Loomis) and Lynda van der Klok (P. J. Soles), that she believes someone is following her, but they dismiss her concerns. Later at her house, Laurie is startled when she sees Michael outside in the yard staring into her room. Michael’s psychiatrist, Dr. Sam Loomis (Donald Pleasence), having anticipated Michael’s return home, goes to the local cemetery only to discover that Judith Myers’ headstone is missing. Loomis meets with Sheriff Leigh Brackett (Charles Cyphers), and the two begin to search for Michael.
That night, Laurie babysits Tommy Doyle (Brian Andrews), while Annie babysits Lindsay Wallace (Kyle Richards) across the street from the Doyle house. When Annie gets a call from her boyfriend Paul asking her to pick him up, she drops Lindsay off at the Doyle house. Annie gets in her car to pick up Paul but Michael, who was hiding in the backseat of her car, strangles her before he slits her throat, killing her. At the Doyle house, while he plays hide-and-seek with Lindsay, Tommy spots Michael carrying Annie’s corpse and tries to tell Laurie, who doesn’t believe in any “boogeyman” that Tommy says he saw. Later that evening, Lynda and her boyfriend Bob enter the Wallace house and have sex in the upstairs bedroom. While downstairs to get a beer for Lynda, Bob is attacked by Michael, who kills him by pinning him to the wall with his knife. Michael then appears in the bedroom doorway, pretending to be Bob in a ghost costume. Gaining no response from him, Lynda becomes annoyed and calls Laurie, just as Michael kills her by strangling her with the telephone cord.
Feeling unsettled, Laurie puts Tommy and Lindsay to bed and goes to the Wallace house, where she discovers the corpses of Annie, Bob, and Lynda when she is suddenly attacked by Michael. She falls down the staircase, but she quickly recovers. Fleeing the house, she screams for help, but to no avail. Running back to the Doyle house, she realizes she lost the keys and the door is locked, as she sees Michael approaching in the distance. Laurie panics and screams for Tommy to wake up and open the door quickly. Luckily, Tommy opens the door in time and lets Laurie inside. Laurie instructs Tommy and Lindsay to hide and then finds the phone line dead and that Michael has gotten into the house through a window. As she sits down in horror next to the couch, Michael appears and tries to stab her, but she stabs him in the side of his neck with a knitting needle and he collapses.
Laurie goes upstairs telling Tommy and Lindsay she killed the “boogeyman”, but Michael reappears in pursuit of her. Telling the kids to hide and lock themselves in the bathroom, Laurie opens the French windows to feign escape and hides in a bedroom closet. Michael punches a hole in the closet door to get to her. However, Laurie frantically undoes a metal clothes hanger and sticks Michael with it in the eye. Michael drops his knife, which Laurie uses to stab him. Michael collapses again and Laurie exits the closet, then tells the children to go find help. Dr. Loomis sees Tommy and Lindsay running away from the house screaming and suspects Michael could be inside. Back inside, Michael gets up and tries to strangle Laurie, but Loomis arrives in time to save her. Loomis shoots Michael six times, resulting in him falling from the second-story patio onto the lawn below. Laurie says to Loomis: “It was the boogeyman”, which Loomis confirms. Despite his seemingly fatal injuries, when Loomis looks down over the balcony he sees Michael’s body has disappeared. The films ends as the audience hears heavy breathing in the background.