The film opens with a man who discovers a buried box and brings it home, where his wife immediately views the object with suspicion. Before long, a disturbing being crawls out of the box and murders her in front of her husband, setting a grim tone that lingers throughout the days that follow.
Years later, Casey, an American seeking to rebuild his life in a quiet British town after the death of his girlfriend in a violent attack, starts a new chapter by taking a job at an antique museum. There he meets Lisa, a coworker who shows him a hidden room filled with items kept off public display because their value is considered dubious. Casey, whose passion for antiquities runs deep, discovers a very old-looking box along with a rare book in the same restricted space. When he touches the box, it activates with a lever pull, revealing a Jack-in-the-box whose puppet is a disturbingly menacing clown—the demon known as the Dollmaker, James Reynard.
That night, Casey and Lisa share a meal and begin to know each other better: Lisa confides that she cares for a sick mother and that she approaches her work with little enthusiasm. Their quiet dinner is interrupted when two thieves break into the museum. One of them mocks the clown, and at that moment the demon bursts forth and kills the intruders. The following day, Casey finds the museum door left ajar but otherwise everything seems normal. An antiques expert arrives, verifies the true value of the Jack-in-the-box, and recounts legends about certain objects being the homes of demons.
As Casey wrestles with insomnia—haunted by the memory of his late girlfriend, who had called him before dying, a call he ignored—nightmares bleed into reality. The next day, a visiting customer is killed by the demon while Casey, overwhelmed by sleepiness, fails to notice in time, and the victim disappears with a heart-wrenching scream that echoes through the halls.
In the days that follow, a street sign bearing the photo of a missing woman—the very one who screamed in the museum—signals that the danger is spreading. The cleaning lady is violently attacked by the demon just after Casey leaves the building. When a policeman questions him about possible supernatural involvement in the disappearances, Casey’s reluctance to deny the strange events raises suspicion. He phones Maurice, a renowned demonologist, for help, but Maurice does not respond.
Casual curiosity gives way to urgent investigation as Casey enters the museum at night and confronts the demon again, who spares him this time. A quarrel erupts with Lisa, who remains convinced that Casey’s experiences are hallucinations born of insomnia. He then tracks down the old owner of the Jack-in-the-box, a man once accused of murdering his wife, and discovers that it was the demon who killed her — and that the owner was unjustly condemned.
Rachel, his boss, fires Casey for his unusual behavior, while Lisa sides with him and confronts Rachel. Casey seeks out Maurice again; the demonologist explains how to lock the creature away, but warns that running away is the safer option. He also stresses that if a tiny piece of the demon remains outside the box, the ritual will fail. A ghostly apparition of Casey’s girlfriend urges him not to run.
Back at the museum, Casey arrives just in time to rescue Lisa, but he cannot save Rachel, who is devoured by the demon. He fights the creature and manages to trap it inside the box, yet a single claw remains outside the lid, a cruel omen. The police arrive and arrest Casey for murder, while Lisa testifies in his favor but finds little credibility with the authorities. Lisa then attempts to bury the box, hoping to end the nightmare, but the demon re-emerges and devours her as well, sealing Casey’s fate in a world where the box’s malevolent legacy continues to haunt them all.
