Ariel Wolfe, Amanda Righetti, Sara Wolfe's younger sister, is dragged into danger when she and her friend Paul, Tom Riley, are kidnapped by art dealer Desmond Niles, Erik Palladino. Desmond forces Ariel to help locate an artifact inside the abandoned Vannacutt Psychiatric Institute—the demon figurine known as the idol of Baphomet.
Inside, they encounter Dr. Richard Hammer, Steven Pacey, and his assistants Kyle, Andrew Lee Potts, and Michelle, Cerina Vincent, who has been secretly aligned with Desmond. Ariel explains the building has been rigged to keep everyone inside for at least 12 hours, and the group splits up to search for the idol. As the search unfolds, the inmate ghosts begin to intervene, slaying Desmond's henchmen and offering haunting glimpses of the inmates’ suffering under the cruel regime of Dr. Vannacutt, the sadistic figure from the 1930s. The visions reveal that the idol has driven Vannacutt to madness and that a revolt by the inmates led to the sanatorium burning down; the ghostly history suggests the idol’s power compels the living and the dead to carry out Vannacutt’s will, rather than the inmates acting of their own accord.
Even after the initial 12 hours elapse, the master locking mechanism re-engages and the building seals itself once more. Ariel escapes but soon learns that Paul has ventured inside to find her, prompting her to re-enter. Desmond’s attempt to claim the idol is derailed when Michelle is killed by Vannacutt, and the remaining group discovers a way out that is blocked by iron bars. The ghost of an inmate guides Ariel toward the location of the idol—the crematorium, the so‑called “heart of the house.”
Ariel, Paul, and Dr. Hammer descend to the crematorium and confront the living flesh constituting the idol’s core. The idol proves indestructible, so Ariel reasons that it must be flushed from the building to free the trapped souls. Desmond ambushes them again, hoping to seize the idol, only to be seized by the ghosts and burned alive after a vision of a patient meeting a similar fate. The idol’s malevolence overwhelms Dr. Hammer, who nearly harms Ariel before the ghostly confrontations reach a fever pitch. Hammer regains clarity, but Dr. Vannacutt’s vengeful specter overwhelms him, killing him. In a final act, Ariel flings the idol into the sewer, the spirits vanish, and the inmates’ ghosts turn their attention toward the sadistic physician, tearing him apart as the building begins to release its hold. Ariel and Paul exit into the light as the site is unsealed.
In a post‑credits scene, a man and a woman on a beach are about to share an intimate moment when the woman senses something beneath the sand. They dig and pull the Baphomet idol into the light, leaving a chilling reminder of the danger that lies beneath.
