Jessica Loren, a rookie police officer on her first assignment, is ordered to take the last shift at a police station before it is permanently closed. Her mother pleads with her not to take the job, as her father, also a police officer, was killed while on duty. After assuring her mother that the shift will be uneventful, she reports for duty. Loren surprises her commanding officer Cohen, who demands that she turn around. He leads her on a brief tour, during which he explains that a HAZMAT team will be there shortly to collect evidence that is difficult to dispose of and that she is not permitted to leave her post. Before he leaves, he gives Loren his phone number in case of emergency.
Loren becomes bored and almost falls asleep, though she wakes when she hears knocking. She does not see anyone at the door but turns around to find a homeless man standing in the hallway; he urinates on the floor and refuses to leave. Loren takes him to a holding cell, where the door suddenly closes and locks behind her. The lights go out, a bloody-faced person surprises her in the door’s window, and she drops her flashlight. An unidentified person picks it up and taunts her. When the lights come back on, the door opens and no one is there.
Loren is further unsettled when she sees chairs and doors moving on their own, ghostly figures appearing out of nowhere, and hears disembodied voices singing. She also receives a series of distress calls from a woman named Monica Young who says she has been taken hostage by a cult; the police dispatcher confirms that all emergency calls have been rerouted to the new station. Loren encounters a loiterer, Marigold, who tells her that she was in a cell when the police brought in an infamous cult, led by the charismatic John Michael Paymon. Marigold says the story told to the public, that the cult was killed at their residence, is untrue; instead, they committed suicide at the station one year ago to the day. Marigold leaves shortly after, humming the song Loren heard earlier.
Further paranormal events at the station reveal that the cult worships the king of Hell, a being also named Paimon, said to reside in Hell before Satan was sent there. Before committing mass suicide, Paymon threatened to come back and destroy everything their arresting officers loved. Ryan Price, a fellow police officer, arrives at the station, and she accuses him of planning all the events as a hazing ritual. Confused, he insists he has come by to check up on her, as he had served with her father. Price confirms that the murderous cult was captured alive and tells her that her father, who died apprehending them, would be proud of her. As he leaves, Loren sees a bullet wound in the back of his head and he disappears.
After seeing several more visions of the cultists and their victims as the night continues, Loren calls Cohen and says that she cannot finish her shift, but repents when he threatens to fire her. The dispatcher tells Loren that Monica was the final victim of Paymon’s cult and died over a year prior. A member of the cult takes Loren hostage, only to commit suicide in front of her. Loren tries to leave the station after evading a deformed girl, but the glass does not break when shot. Her dead father calls her and demands justice for his death, and she returns to the holding cell to find the homeless man’s body hanged. Her father calls again to warn her of several cult members assaulting the station, and she kills them. As she shoots the last one, Cohen shoots her from behind. In a moment of clarity, she realizes that she has murdered the entire HAZMAT team. Cohen calls for medical assistance, and Loren is approached by the spirits of Paymon and his cultists as she bleeds out.