An ordinary day turns sinister when a man unearths a buried box and brings it home. His wife grows uneasy, sensing something off about the object. That night, a grotesque entity crawls from the box and kills her right before her husband’s eyes. Years later, Casey, an American newcomer, arrives in a quiet British town hoping to rebuild his life after the death of his girlfriend in a violent attack.
At a local antiques museum, he begins work and meets Lisa Cartwright, a colleague who shows him a back room where items of dubious value are stored away from the public display. Drawn to a rare book, he also discovers an unusually old-looking box tucked among the shelves. When he touches it, the object stirs to life, and the puppet inside—the creepy Jack of the Jack-in-the-box—emerges as a disturbing clown and seems to fix its gaze on him.
That evening, over a quiet dinner with Lisa, the couple’s budding rapport is interrupted by a break-in at the museum. Two thieves enter; one dares to insult the clown in the box, and in response a monstrous force erupts from the jack, tearing the thieves apart. The next day, Casey finds the museum door ajar but otherwise nothing seems out of the ordinary. An expert in ancient artifacts arrives to assess the Jack-in-the-box and confirms what legends have long claimed: some objects are believed to be the home of demons. The man behind the box, a deeply unsettling presence, hints at powers beyond explanation.
Casey’s sleep is fractured by memories of his girlfriend, who had called him before dying; he feels guilt that he ignored the call. A new client later visits the museum, but Casey, exhausted, pays little attention and the demon claims the woman in a swift, heart-wrenching moment. In the days that follow, Casey notices a poster bearing the missing woman’s photo—the same victim he heard scream at the museum. The next day, the cleaning lady is brutally attacked by the demon. When a policeman named Detective Martin interrogates Casey, he hints at supernatural forces behind disappearances, and Casey’s reluctance to cooperate only deepens the suspicions.
Frustrated and scared, Casey decides to confront the mystery and returns to the museum at night, where the demon confronts him but spares him for the moment. After a tense exchange with Lisa—who believes his story is a product of insomnia—Casey tracks down the old owner of the Jack-in-the-box, a figure once accused in a wife’s murder. The encounter reveals that the demon was indeed responsible for the wife’s death, even though the owner was unjustly condemned.
The tension peaks when Casey is called to the office of his boss, Rachel, who fires him for his erratic behavior. Lisa is furious with Rachel for the decision, and Casey seeks help from Maurice, a renowned demonologist. Maurice explains how to seal the demon back inside the box, but warns that the ritual will fail if even a tiny fragment of the demon remains outside. A vision of his girlfriend’s ghost urges him to stay and act, and Casey heads back to the museum to intervene.
In a desperate confrontation, Casey fights the demon and manages to trap it inside the box, though a single claw remains outside the lid. The police arrive and arrest him for murder, while Lisa volunteers to testify in his favor but struggles to be believed. Lisa then attempts to bury the box to end the nightmare, but the demon escapes once more and devours her as the case seems to close in uncertainty.
