In 1963, on Halloween night in the small fictional town of Haddonfield, Illinois, six year-old Michael Myers inexplicably stabs his older sister Judith to death with a kitchen knife in their home, and is incarcerated at Smith’s Grove sanitarium. Fifteen years later, on October 30, 1978 Michael’s psychiatrist Dr. Samuel Loomis and his colleague Marion Chambers arrive at the sanitarium to escort Michael to court. Michael escapes by stealing their car and returns to Haddonfield, killing a mechanic for his coveralls, as well as stealing a white mask and knives from a local hardware store.

The next day on Halloween, when high school student Laurie Strode drops off a key at the Myers’ home so her father can sell it, Michael stalks her throughout the day which she notices, but her friends Annie Brackett and Lynda Van der Klok dismiss her concerns. Loomis arrives in Haddonfield in search of Michael, and finds Judith’s tombstone missing from the local cemetery. He meets with Annie’s father, Sheriff Leigh Brackett, and they both investigate Michael’s house, where Loomis tells Sheriff Brackett of the danger Michael poses. Sheriff Brackett, while doubtful of Michael’s danger, patrols the streets, while Loomis waits at the house, expecting Michael to return.

Later that night, Laurie babysits Tommy Doyle, while Annie babysits Lindsey Wallace just across the street, unaware that Michael has followed them. When Annie’s boyfriend, Paul, calls her to come and pick him up, she takes Lindsey over to the Doyle house to spend the night with Laurie and Tommy. Before Annie can leave in her car, Michael, who hid in the back seat, strangles her and slits her throat. Soon after, Lynda and her boyfriend, Bob Simms, arrive at the Wallace house. After having sex, Bob goes downstairs to get a beer for Lynda, where Michael stabs him. Michael then poses as Bob in a ghost costume and confronts Lynda, who teases him to no effect. Annoyed, Lynda calls Laurie, but Michael strangles her with the telephone cord just as Laurie picks up. Meanwhile, Loomis discovers the stolen car and begins searching the streets.

Suspicious, Laurie goes over to the Wallace house, and finds the bodies of Annie, Bob, and Lynda, as well as Judith’s headstone. She cowers in a hallway, where Michael suddenly appears and attacks her. Barely escaping, Laurie flees back to the Doyle house; Michael gets in, but she manages to fend him off long enough for Tommy and Lindsey to escape. Loomis sees the two children fleeing the house and goes to investigate, finding Michael and Laurie fighting upstairs. Loomis shoots Michael six times causing him to fall off the balcony to an apparent death, but when Loomis goes to check, he finds Michael’s body missing, implying he survived. Unsurprised, he stares off into the night while Laurie begins sobbing in terror.