Stage hypnotist/magician "The Great Vorelli" and his dummy Hugo perform before a packed audience in London. The audience observes tension between the magician and Hugo, which Vorelli keeps in a locked cage between performances. American reporter Mark English is assigned a story on Vorelli, and solicits his girlfriend Marianne Horn, a wealthy heiress, to go with him to another show.
At the show, Vorelli asks for a volunteer. Mark encourages Marianne to go up. Vorelli hypnotizes her and makes her dance the twist. During Vorelli's ventriloquism act, Hugo gets up from his chair and walks around, seemingly under his own power. Mark, wanting a closer look at Hugo to determine how this trick is performed, gets Marianne to invite Vorelli to her aunt's charity ball.
While Vorelli performs his ventriloquism at the ball, Hugo takes a knife from the buffet table and tries to stab Vorelli, only stopping when Vorelli focuses all his will. The guests assume this is part of Vorelli's act. Mark secretly examines Hugo, and finds he is a simple dummy, without clockwork mechanisms, a space for an operator, or any other feature that might allow him to walk on his own. The night of the ball, Vorelli stays at the mansion of Marianne's aunt, where he rapes Marianne after using his power to subdue her will. Hugo appears in Mark's room and pleads "Help me ... 1948 ... Berlin" before disappearing.
Marianne falls into a semi-coma that the doctors cannot alleviate. In one lucid moment, she tells Mark that "He keeps calling me" and "Make him stop". Mark realizes her state is the result of being hypnotized by Vorelli, and begins an investigation into Vorelli's past. Through a colleague, Mark discovers that Vorelli was a medical doctor who dabbled in Eastern magic and was disbarred. The colleague guides Mark to a former assistant of Vorelli's who lives in Berlin, named Mercedes. She tells Mark that another assistant, Hugo, worked for Vorelli in 1947, and was hypnotized into a state where he could not feel pain as part of their act. Mercedes would catch the two in strange conferences. One night, Vorelli stabbed Hugo on stage, and this time Hugo reacted with pain. Hugo was comatose for three months, during which Vorelli transferred Hugo's soul into the dummy, resulting in his death. The death was ruled an accident, and no one believed Mercedes's story, despite a theatre worker testifying he saw the dummy move immediately after Hugo screamed in pain.
Mark suspects Vorelli deliberately hypnotized Hugo to die from the knife wound, but his concerns over Vorelli are greatly assuaged when he hears that Marianne has awoken from her comatose state.
Vorelli's current assistant and lover, Magda, is outraged at his rape of Marianne and threatens to go to the police. Vorelli taunts Hugo into murdering Magda with a knife when Vorelli is visiting with stage crew elsewhere. Vorelli then hires a new, younger assistant named Grace whom he also puts under his hypnotic control.
Vorelli visits Marianne in her home and hypnotizes her into agreeing to marry him. Vorelli confides to Hugo that he plans to marry Marianne in Spain and transfer her spirit into another doll before letting her body die and inheriting her wealth. He opens Hugo's cage, intending to discipline him due to his recent rebellions. Instead, Hugo smashes the face of the doll intended for Marianne, and attacks Vorelli. The two struggle, their two souls interacting, until Vorelli finally locks Hugo back in his cage. Mark enters the room. Vorelli speaks in Hugo's voice and tells Mark that Hugo has now transferred his soul into Vorelli's body and vice versa and that Marianne's hypnotized state is broken. From Hugo's former puppet body, Vorelli begs for help from Mark.
Vorelli’s confrontation with Hugo culminates in a reversal—Mark discovers the secret behind Hugo, and the plans for soul transference are upended, leaving the power in unexpected hands.
The ending reveals the full extent of Vorelli's depravity and the supernatural mechanics at play, as the dummy Hugo and Vorelli face off over the fate of Marianne. Soul transference is the film's core horror, and with Vorelli's downfall, the theme of control is flipped—the manipulator becomes the manipulated. It's a fittingly eerie coda to a film that never lets its audience fully trust the puppeteer.
