The film opens with a man discovering a buried box in the yard, bringing it home despite his wife’s growing irritation with the eerie object. Before long, a sinister presence erupts from the box and, in a brutal moment, kills the wife right before the husband’s eyes.
Years later, Casey, an American trying to rebuild his life after the death of his girlfriend, arrives in a quiet British town. He takes a job at an antiques museum, where he meets Lisa, a coworker who shows him a room filled with objects deemed too dubious for display. Among them, Casey—whose lifelong passion for antiques runs deep—uncovers a very old-looking book and a peculiar box. When he touches the box, it springs to life. Inside sits a Jack-in-the-box puppet, Jack whose unsettling clown visage sets a chilling tone.
That evening, as Casey and Lisa eat together and get to know one another, two thieves break into the museum. One taunts the clown, and in response the box erupts with a monstrous being that kills the intruders. The next day, Casey finds the museum as quiet as ever, save for the recent disturbance. An expert in ancient objects arrives and verifies the true value of the Jack-in-the-box, then mentions legends about some boxes hosting demons. The memory of Casey’s late girlfriend—who had called him just before she died, a call he ignored—haunts him and fuels a creeping guilt.
The following day, a customer arrives at the museum, but Casey is too sleepy to give proper attention. The demon claims another victim, and Casey only hears a heart-wrenching scream before the woman vanishes. In the days that follow, a street-side sign displays the missing photograph of the same woman; the next morning the cleaning lady is attacked by the demon. During a police interrogation, Casey’s questions about the supernatural draw suspicion.
Casey then contacts Maurice, a well-known Demonologist, to seek answers. The expert’s response is elusive, and Casey resolves to confront the mystery himself. He sneaks into the museum at night and encounters the demon, who spares him, creating a bond of terror between them. After a tense confrontation with Lisa, who believes his visions to be the product of insomnia, Casey tracks down the museum’s old Jack-in-the-box owner. The owner, once accused of his wife’s murder, confirms that the demon killed the wife and that the truth was ignored.
With pressure mounting, Casey is summoned and ultimately fired by his boss, Rachel. The decision sparks further conflict with Lisa, who sides with him in private but cannot sway Rachel. Casey then travels to Maurice for guidance again. The demonologist explains how to re-trap the demon and cautions him to leave no trace of the creature outside the box, lest the ritual fail. Though the ghost of his girlfriend pleads with him to stay, Casey returns to the museum just in time to rescue Lisa, but Rachel has already fallen prey to the monster.
A brutal struggle ensues as Casey fights the demon and succeeds in sealing it back inside the box, though a single claw remains outside. The police arrive and arrest Casey for murder, while Lisa testifies in his favor but remains unconvinced by the jury. Lisa then tries to bury the box to end the terror, but as Casey faces interrogation and a crime-scene photo showing the missing claw, the demon re-emerges and devours her.
