Haddonfield, Illinois. October 31, 1978.
The film begins where its predecessor left off, with Laurie Strode sending the two kids she’s babysitting to run and get help after the home has been attacked by Michael Myers. After the kids run off screaming, she is attacked by Myers once again, and is rescued when Dr. Sam Loomis arrives and shoot Myers off of the upstairs balcony.
From here, the film is entirely new. Loomis rushes out to the lawn and finds that Michael has vanished, leaving behind only a bloody imprint in the grass. He instructs a neighbor to phone the news to the police and runs off in search of Michael again.
Similarly to the opening credits of the previous film, the opening credits show a jack-o-lantern which slowly opens to reveal a human skull. Afterward, Michael Myers lurks in the alleys nearby, watching Dr. Loomis from a distance. Loomis meets up with Sherrif Leigh Brackett and they drive off in another direction in search of Myers. Myers then sneaks into a nearby house and steals a kitchen knife from an elderly woman named Mrs. Elrod.
Across the street, Myers sees a young girl by the name of Alice outside and stalks her. Inside, Alice is speaking on the phone with a friend who informs her of the murder in Haddonfield. Alice switches on the radio and is shocked at the news when suddenly, the back door creaks open behind her. Afraid, Alice ventures into the living room where Myers jumps out and stabs her.
Paramedics arrive at the Doyle house and Laurie is wheeled away on a stretcher by two EMTs, Budd and Jimmy. She appears to be in shock, and she begs them not to put her to sleep. Jimmy is younger and takes an immediate interest in Laurie, concerned and obviously distressed that someone close to him in age has been attacked in Haddonfield. They take her to Haddonfield Memorial Hospital, where she is tended to by the half-drunk Dr. Mixter, head nurse Mrs. Alves, and RN Jill.
Meanwhile, Dr. Loomis and Sheriff Brackett circle the neighborhood and happen upon a young man wearing a white mask similar to the one worn by Michael Myers. They begin to pursue him on foot, but an approaching patrol car crashes into him and then into the side of a van. The violent explosion incinerates the body. One of the approaching patrolmen, Deputy Hunt, has come to tell Sheriff Brackett the bad news that his daughter is among the dead that have been discovered in the wake of Myers’s rampage. They drive off to the location so that Sheriff Brackett can identify her.
Sheriff Brackett, in shock at finding his daughter dead, blames Loomis for allowing Michael to escape and departs for his home, leaving Deputy Hunt in charge. Loomis convinces Deputy Hunt that Michael Myers could possibly still be alive, since they are uncertain who exactly was struck and killed by the patrol car.
Another young nurse, Karen, who works in the nursery, arrives at the hospital after staying too late at a Halloween party. She is sharply reprimanded by Mrs. Alves, even as they are both unaware that Michael Myers is lurking in the hospital and watching them.
Jimmy schemes to be alone with Laurie, both to console her and get closer to her. He reveals to her that the crazy man who was pursuing her earlier was Michael Myers, who escaped from the mental hospital the previous night. Laurie is confused and does not understand why she seems to have been targeted by Myers. Mrs. Alves interrupts and sends Jimmy away, then discovers that the phones are suddenly out of service. The security guard, Mr. Garrett, explores the basement and rear of the hospital to search for the trouble and after he searches one of the closets, Michael suddenly leaps out and kills him by bashing him over the head with the back of a hammer.
Across town, Dr. Loomis and Deputy Hunt investigate a break in at the local elementary school and discover that Michael Myers has been there. A child’s drawing of a family has a butcher knife sticking in the image of the sister, and the word “Samhain” has been scrawled in blood on the chalk board. Loomis remarks on the Celtic origins of the word, and is surprised when Marion Chambers, from the previous night’s events, appears. Nurse Chambers has been sent with a Marshall to return Dr. Loomis to Smith’s Grove, on orders from the governor. Dr. Loomis is forcibly taken away, while Deputy Hunt promises to find Michael.
Meanwhile, Michael continues to murder the staff at the hospital. Budd and Karen attempt to make love in a hospital therapy pool, but Myers turns up the temperature until Budd gets out to check it. After strangling Budd to death, he approaches Karen from behind and seizes her, holding her under the scalding hot water until the flesh has peeled from her face.
Laurie has a reaction to her medication and goes into shock. When Janet goes to get Dr. Mixter, she finds him dead in his private office with a needle thrust into one eyeball. In a similar fashion to his reveal to Laurie, Michael appears behind Janet and stabs a needle into her temple.
Unable to find anyone in the hospital, Jimmy starts to wonder around while Jill responds a call from a patient. With the coast clear, Michael enters Laurie’s room and stabs at her bed, only to discover she’s no longer under the sheets. When Jill returns, she finds the room empty. Hindered by a cracked ankle and dazed from the drugs, Laurie limps through the now-quiet halls of the hospital, looking for a safe place to hide.
Jimmy reappears and sends Jill off to get help, telling her to drive into town and bring back the police. After she leaves, Jimmy finds the body of Mrs. Alves in a nearby operating room, tied to a table with an IV tube in her arm having drained her blood out onto the floor. He goes to leave, but instead slips in the blood and falls on his back, slamming his head down on the floor and rendering him unconscious.
Jill attempts to leave the hospital, but finds all of the cars in the parking lot have been disabled with the tires slashed and the engines tampered with. After running back into the hospital, she sees Laurie in a hallway and tries to reach her. Before Laurie can respond, Michael appears behind Jill and kills her by stabbing a scalpel into her back. Laurie witnesses it and is terrified into action; she limps through the halls with Michael in pursuit. Rushing down a stairwell and into the basement, Laurie corners herself in a boiler room and and discovers the body of Mr. Garrett. With Michael looming closer, Laurie must climb through a small window near the ceiling to escape and just narrowly avoids Michael slashing her ankles. After nearly cornering her again in an elevator, Laurie escapes and rushes into the parking lot, hiding in a parked car.
In the Marshal’s car, Marion has a discussion with Dr. Loomis where she reveals information that he did not previously know; Laurie Strode is actually the sister of Michael Myers, born two years before Michael’s original crime and adopted after the death of the parents two years after. Loomis immediately understands that Michael is after Laurie for a reason and commandeers the Marshal’s car, pulling a gun on him and forcing him to make for the hospital.
In the parking lot of the hospital, Laurie sees a dark shape open the car door and get inside – but it’s only Jimmy. He tries to start the car and cannot; Laurie tries to talk to him but he once more passes out onto the steering wheel. The care horn begins to blare, and Laurie panics now that her hiding spot has been revealed. She tries to leave the car and falls in the parking lot, still dazed. As she lays there, the Marshal’s car pulls up and Dr. Loomis, Marion, and the Marshal get out and go into the hospital. Laurie cannot cry out until it’s too late and they are inside, pulling the doors shut behind them.
Finally able to get to her feet, Michael appears across the parking lot and slowly begins to walk toward her. Laurie makes it to the doors in time to bang on them and attract the attention of the others. They let her inside just in time; seconds later, Michael walks right through the closed glass doors. After Dr. Loomis shoots him with his gun, he falls over, seemingly dead. Dr. Loomis sends Marion out to the Marshal’s car to use the radio for summoning the police, before turning his attention towards comforting Laurie. The Marshal goes to investigate Michael’s body, against Dr. Loomis’s protests, and finds his throat slit with the scalper after Michael rises up again.
Dr. Loomis and Laurie escape, finding themselves in an operating room. As Michael begins to break down the door, Dr. Loomis gives Laurie his gun, which only has two shots left in it. Michael finally breaks through the door and stabs Dr. Loomis in the stomach. Laurie tries to stop Michael by calling his name, which works only for a moment before he makes his way towards her again. She makes a perfect shot and shoots him in both of his eyes, causing Michael to begins swinging his scalpel wildly. Loomis struggles to his feet and begins releasing oxygen into the room from the many tanks on the walls, causing a noise that makes Michael change his path. Laurie follows suit and the two take turns confusing Michael until the doctor tells her to run. She bolts from the room and runs down the hall as Loomis ignites his cigarette lighter and causes a huge explosion. Michael emerges from the flames and keeps walking toward Laurie, until the flames finally consume him and he falls.
As the sky lightens into morning, Laurie is taken outside the hospital and placed into the back of an ambulance, as reporters, authorities and other onlookers watch. She stares blankly as the ambulance pulls away, seemingly in shock, but alive. The final image in the film is the face of Michael’s burning corpse.
Dead… for now.