On Halloween in Haddonfield, Illinois, having already shown signs of psychopathic tendencies, ten-year-old Michael Myers () murders a school bully named Wesley ). Later that night, Michael murders his older sister Judith ), his mother's boyfriend Ronnie ), and Judith's boyfriend Steve ). Only his baby sister, Angel Myers, is spared. After one of the longest trials in the state's history, Michael is found guilty of first degree murder and sent to Smith's Grove - Warren County Sanitarium under the care of child psychologist Dr. Samuel Loomis .
Michael initially cooperates with Dr. Loomis, claiming no memory of the killings; his mother, Deborah ), visits him regularly. After a year, Michael becomes fixated on his papier-mâché masks, closing himself off from everyone, even his mother. When Michael kills a nurse ) as Deborah is leaving from one of her visits, she can no longer handle the situation and commits suicide. For the next fifteen years, Michael continues making his masks and not speaking to anyone. Dr. Loomis, having continued to treat Michael over the years, attempts to move on with his life and closes Michael's case. Later, while being prepared for transfer to maximum security, Michael escapes Smith's Grove, killing the sanitarium employees and a truck driver (Ken Foree) for his overalls, and heads to Haddonfield. On Halloween, Michael arrives at his old home, now abandoned, and recovers the kitchen knife and Halloween mask he stored under the floorboards the night he killed his sister.
The story shifts to Laurie Strode ), and her friends Annie Brackett and Lynda Van Der Klok () on Halloween. Throughout the day, Laurie witnesses Michael watching her from a distance. That night, she heads to the Doyle residence to watch their son Tommy ). Meanwhile, Lynda meets with her boyfriend Bob at Michael's childhood home. Michael appears, murders them, and then heads to the Strode home, where he murders Laurie's parents, Mason (Pat Skipper) and CynthiaHaving been alerted to Michael's escape, Dr. Loomis comes to Haddonfield looking for Michael. After obtaining a handgun, Loomis attempts to warn Sheriff Brackett (Brad Dourif) that Michael has returned to Haddonfield. Brackett and Dr. Loomis head to the Strode home, with Brackett explaining along the way that Laurie is actually Michael Myers' baby sister.
Meanwhile, Annie convinces Laurie to babysit Lindsey Wallace (, a girl Annie is supposed to be watching, long enough so she can have sex with her boyfriend Paul. Annie and Paul return to the Wallace home; during sex, Michael kills Paul and attacks Annie. Bringing Lindsey home, Laurie finds Annie on the floor, bloodied but alive, and calls 911. She is attacked by Michael, who chases her back to the Doyle home. Sheriff Brackett and Loomis hear the 911 call and head to the Wallace residence. Michael kidnaps Laurie and takes her back to his home. Michael approaches Laurie and tries to show her that she is his younger sister, presenting a photo of the siblings with their mother. Unable to understand, Laurie grabs Michael's knife and stabs him before escaping the house; Michael chases her, but is repeatedly shot by Dr. Loomis. Laurie and Loomis are just about to leave when Michael grabs Laurie and heads back to the house. Loomis intervenes, but Michael attacks him by squeezing Loomis's skull with his hands. Laurie takes Loomis's gun and runs upstairs; she is chased by Michael, who, after cornering her on a balcony, charges her head-on and knocks both of them over the railing. Laurie finds herself on top of a bleeding Michael. Aiming Loomis' gun at his face, she repeatedly pulls the trigger until the gun finally goes off just as Michael's hand grips Laurie's wrist.
