Jessie Holden [Trish Coren] is invited by her boyfriend, Kevin [Jilon VanOver], and two friends, Freddy [Josh Holt] and Marie [Nicole Rayburn], to spend one Halloween night at Santa Mira Hospital, a building long abandoned after a fire. Their friend Emmett [Happy Mahaney] is there, busy with props and his loyal dog Dutchess [Dutchess Botti-Smith], ready to scare Jessie. The mood turns tense when a ghost suddenly attacks Emmett and Dutchess, underscoring the hospital’s chilling history. At the same time, Allan [Michael Samluk] is searching for his sister Meg [Rachel Melvin], who disappeared with her friend Caitlin [Jennifer Amy Decker] inside the same place days earlier. Allan’s father’s old friend, Arlo Ray Baines [Dig Wayne], a former actor turned cop, initially refuses to join but eventually decides to tail Allan as he moves deeper into the dark corridors. The encounter on the third floor intensifies when Allan discovers Dutchess horribly mutilated, only for the dog to mysteriously rise again and attack him until he shoots her, a grim sign that something supernatural is at play. Entering the third floor, Allan reunites with Meg, who explains she got separated from Caitlin during the hospital’s eerie maze of rooms and stairwells.
Arriving inside the hospital, the four friends split up to search for Emmett. In a troubling turn, Kevin cheats on Jessie by having sex with Marie, a betrayal that unsettles the group’s fragile trust. Jessie, who is spiritually aware, shares with Freddy the story of her late mother, who died from illness years ago but continues to call to Jessie every Halloween. The search leads them to Emmett, who is terrified in the locker room. When the group regroups, Emmett suddenly attacks again, forcing Allan and Meg to intervene. Meg uses Allan’s gun to shoot Emmett to bits, and she warns the others that everyone who dies in the hospital is reanimated—Emmett and Dutchess are among them. A paranoid Kevin, convinced that Marie is turning into a ghost, grabs Allan’s gun and shoots her dead, amplifying the sense that the hospital is turning people into something other than themselves.
Because Meg refuses to use the elevator, the party splits: Jessie, Kevin, and Freddy haul Marie’s body toward the elevator, while Allan and Meg take the stairs. The trio are transported to the third floor, where Jessie experiences a series of ominous premonitions about a child murderer named Jacob. Jacob had been confined to this floor, a wing reserved for mental patients, yet he managed to kill a little girl on the premises. When he tries to flee by setting the hospital on fire as a distraction, Nurse Russell [Dee Wallace] discarded the door keys, trapping them in the blaze. Jacob has been killing and reanimating people to possess them, but so far he has failed to find a suitable host. As they explore, Kevin disappears from sight.
Jessie and Freddy regroup with Allan, Meg, and Arlo [Dig Wayne], who has just killed the reanimated Caitlin and resolved to save them all. They discover Kevin trapped by the little girl’s spirit and manage to free him, but Freddy has died and been reanimated, forcing Arlo to shoot him. The remaining survivors push toward the stairs, hoping to escape, but Jessie realizes that Jacob will not be forced out so long as he holds power over the living. She devises a plan to unbalance him by impersonating Nurse Russell, using fear to sever his grip on the living world. The ruse works, though Jacob retaliates by animating Marie to kill Kevin. Kevin rises once more, but it becomes clear that Jacob has taken him as a vessel. At the same time, Meg reanimates as well, yet she is possessed by Nurse Russell and retains enough control to resist Jacob’s influence. Russell orders the others to leave as she confronts and subdues Jacob in a dramatic showdown.
Jessie, Allan, and Arlo manage to escape the hospital, stepping into the bright noon light of All Saints’ Day outside. Back inside, Jacob promises to escape again, but Russell calmly states that he will have to get through her first before she can switch off the lights for good.
