Serial killer Michael Myers stalks and kills the people of Haddonfield, Illinois on Halloween.
On Halloween night 1963, in the town of Haddonfield, Illinois, six-year-old Michael Myers (Halloween), dressed in a clown costume, stabs his older sister Judith to death with a kitchen knife in their home. Fifteen years later, on October 30, 1978, Michael's psychiatrist, Samuel Loomis, and his colleague, Marion Chambers, arrive at Warren County's Smith's Grove Sanitarium to escort Michael to court. Michael escapes the sanitarium, stealing Loomis' car in the process, and returning home to Haddonfield.
On , Michael Myers (Halloween) is shot and falls off a balcony. Taking shelter to recover from his injuries, Michael steals a kitchen knife from the home of an elderly couple, and kills a teenage girl living next door. Laurie Strode, who narrowly avoided being killed by Michael earlier in the night, is taken to Haddonfield Memorial Hospital, while Michael's psychiatrist, Samuel Loomis, continues his pursuit of his patient. Accompanied by Sheriff Leigh Brackett, Loomis mistakes teenager Ben Tramer for Michael, when Ben gets hit by a squad car which pins him to another vehicle that bursts into flames, leading to his death. Upon learning his daughter Annie Brackett was killed by Michael, Sheriff Brackett goes off duty to inform his wife, leaving Deputy Gary Hunt in charge to help Loomis.
On October 23, in Northern California, shop owner Harry Grimbridge is chased along a barren road by mysterious men in suits. He makes it to a gas station, clutching a jack-o'-lantern mask. He is driven to the hospital by station attendant Walter Jones. At the hospital, Harry is placed in the care of Dr. Dan Challis, a hardworking physician struggling with the relationships between him and his ex-wife, as well as his children. That night, another man in a suit enters Harry's hospital room, murders him, then Self-immolation. The next day, Dan is confronted in a bar by Harry's daughter, Ellie. He tells her about the strange events of the night before and shows her the mask her father was holding when he was admitted to the hospital. Along with Ellie, Dan traces the mask back to Silver Shamrock Novelties, a company based in the town of Santa Mira, California. The motel manager explains that Conal Cochran and his company, Silver Shamrock Novelties, which produces wildly popular latex jack-o-lantern, witch and skeleton masks for Halloween, are responsible for the town's prosperity. While signing the motel register, Dan learns that Harry stayed at the same motel. Other motel guests include shop owners Marge Guttman and Buddy Kupfer, Buddy's wife, Betty, and their son, Little Buddy, who all have business at the company's factory.
On October 30, 1988, Michael Myers, who has been in a comatose state for ten years since the explosion at Haddonfield Memorial Hospital, is transferred from the Ridgemont Federal Sanitarium to Smith's Grove Sanitarium by ambulance. En route, Michael wakes from his coma when he learns that his sister Laurie Strode, who died in a car accident, has a daughter, Jamie Lloyd. He attacks the ambulance personnel, and the vehicle crashes. Michael makes his way to Haddonfield in search of his niece. Dr. Sam Loomis goes to Haddonfield after learning that Michael has escaped transfer and gives chase. He arrives at a gas station, where he comes across Michael who has killed a mechanic for his clothes, along with a clerk. Michael escapes in a tow truck and causes an explosion, destroying Loomis's car in the process and disabling the phones by severing the telephone wires. Loomis is then forced to catch a ride to Haddonfield.
On , Michael Myers is shot and falls down a Shaft mining. Escaping into a nearby creek, he stumbles upon a local hermit and falls into a coma, placing him in the hermit's care, and being nursed back to health by him. One year later, on October 30, 1989, Michael awakens, kills the hermit, and returns to Haddonfield to find his niece Jamie Lloyd again, who narrowly avoided being killed by Michael the year before.
On , Michael Myers and his niece Jamie Lloyd are abducted from the Haddonfield Police Station. Six years later, on October 30, 1995, Jamie has been impregnated and her infant is born, being taken away by the "Man in Black", the leader of a Druid-like cult. Later, a midwife helps Jamie escape with her baby but is killed by Michael by impaling the back of her skull into a protruding sharp metal spike high on the wall. Jamie and her baby flee in a stolen pick-up truck, with Michael in pursuit. Meanwhile, Dr. Sam Loomis has retired and moved to a hut on the outskirts of Haddonfield, where he lives as a hermit. He is visited by his friend Dr. Terence Wynn, the chief administrator of Smith's Grove Sanitarium, where Michael had been incarcerated as a boy; Wynn asks Loomis to return to Smith's Grove. They overhear Jamie's plea for help on a local radio station, when she makes a call to Loomis, only to be ignored by the radio D.J. Barry Simms. Michael catches up with Jamie, and she crashes the truck into an old barn. He kills Jamie by impaling her on a corn threshing machine and turning it on, disemboweling her, but finds that her baby is not in the truck.
On October 29, 1998, in Langdon, Illinois, Dr. Loomis' former colleague, Marion Chambers (Nancy Stephens) discovers that her house has been burglarized. Her teenage neighbor Jimmy (Joseph Gordon-Levitt) and his friend Tony (Branden Williams) search the house and call the police. Marion discovers that the file on Laurie Strode (Jamie Lee Curtis), Michael Myers' sister, is missing. She rushes over to warn Jimmy and Tony only to find them both dead. Michael Myers (Halloween) (Chris Durand), who had been presumed dead for the past twenty years, appears and attacks Marion before slitting her throat, killing her. The police arrive as Michael leaves the house with Laurie's file.
On , Laurie Strode accidentally kills a paramedic with whom her brother Michael Myers (Halloween) had swapped clothes. A guilt-ridden Laurie becomes an inmate at the Grace Andersen Sanitarium, where the nurses believe her to be catatonic. Instead, she is preparing for Michael's return. Three years later, on October 31, 2001, Laurie, pretending to be heavily medicated, prepares herself for the inevitable confrontation with her brother. Michael breaches the facility and kills two security guards, as he makes his way to Laurie. After a chase, Laurie lures Michael on to the institution's rooftop. Although he falls into her trap, Laurie's fears of again killing the wrong person get the better of her; when she tries to remove his mask, Michael kills Laurie by stabbing her and throwing her off the roof. Michael returns home to his abandoned childhood house.
On October 29, 2018, Michael Myers (Halloween), who has been in Warren County Smith's Grove Sanitarium for forty years since his killing spree in Haddonfield, is being prepared to be transferred to a new facility. True-crime podcasters Aaron Korey and Dana Haines interview Michael's psychiatrist Dr. Ranbir Sartain, a former student of Samuel Loomis until he died, before meeting with Michael in hopes of gaining some insight into his past actions. Aaron brandishes the mask that Michael wore in 1978, at him, to no effect.
The saga of Michael Myers and Laurie Strode continues in the next thrilling chapter of the Halloween series.