Devil Doll begins with a performance by hypnotist/magician "The Great Vorelli" (Bryant Haliday) and his dummy Hugo before a packed audience in London. The audience observes tension between the ventriloquist and his dummy. American reporter Mark English (William Sylvester) becomes fascinated with Vorelli while attending the performance. English solicits his girlfriend Marianne Horn (Yvonne Romain) to go with him to another show. From the beginning, the film drops strong hints that the dummy, Hugo, is actually alive and mobile.
At the following show, Vorelli asks a member of his audience onto the stage. When no one volunteers, English encourages Horn to go up. Vorelli succeeds in hypnotizing her and making her dance the Twist with an uncredited Ray Landor, "expert Twist dancer". Horn is left partially hypnotized by Vorelli, who recognizes her name as that of a wealthy heiress. English, inexplicably wanting to do a story on Vorelli because of his unique powers, gets Horn to invite Vorelli to her aunt's charity ball. Vorelli has already decided to go to the ball, having read about it in the newspaper and seeing it as an opportunity to seduce the rich Horn.
The night of the ball, Vorelli stays at Horn's aunt's mansion. He calls Horn to his bedroom where he seduces her after using his power to subdue her will. In the meantime, Hugo miraculously appears in English's room and asks English to help him. Hugo repeats "1948" and "Berlin" before disappearing. The next day, English begins an investigation into Vorelli's past. Meanwhile, Horn falls into a semi-coma that the doctors cannot penetrate. In one lucid moment, she tells English that "He keeps calling me" and "Make him stop". It is not clear whether English makes the connection.
Through a colleague, English discovers that Vorelli had once been a medical doctor who dabbled in mysterious Eastern magic and was disbarred for an unknown reason. The colleague traces Vorelli to Berlin from 1947 onward and uncovers the story of a former assistant to Vorelli. English travels to Berlin to interview her. Vorelli's former assistant claims that she and another assistant, "Hugo", had worked for Vorelli in 1947 and 1948. In their act, Vorelli would hypnotize Hugo into a state where he could not feel pain: Vorelli would stab a knife into Hugo, who did not respond and never appeared injured. The female assistant says that over time, she would catch Vorelli and Hugo in strange conferences. One night, Vorelli killed Hugo on stage and simultaneously transferred Hugo's soul into a dummy off stage. Vorelli was cleared of all charges due to the risky nature of Hugo's job. No one believed the female assistant's story. It's unclear how she knew the details of what happened.
Vorelli's present-day assistant appears periodically through the film. She helped Vorelli get his start on the stage and they are still lovers. She becomes jealous of Vorelli's relationship with Horn. Vorelli either manipulates or taunts Hugo into murdering his lover/assistant when Vorelli is visiting with stage crew elsewhere. The murder left unsolved, Vorelli immediately hires a new, younger assistant whom he also puts under his physical and sexual control. Meanwhile, during English's trip to Berlin, Vorelli visits the now-recovered Horn in her home. She is still hypnotized. He tells her to announce that she loves him and is going to marry him. Vorelli confides to Hugo (the dummy) that he plans to take Horn to Spain, marry her, and transfer her spirit into a companion doll for Hugo before letting her body die.
Hugo escapes from his cage, smashes the face of the female doll intended for Horn, and attacks Vorelli. Vorelli seemingly succeeds in wrestling the irate Hugo back into his cage just as Mark English enters the room. "Vorelli" speaks in Hugo's voice and tells English that he (Hugo) has now transferred his soul into Vorelli's body and vice versa. From Hugo's former body, Vorelli begs for help from English, who does not respond as the film ends.
Devil Doll was featured in an October 1997 episode of Mystery Science Theater 3000. The episode was released on DVD by Shout! Factory on 9 November 2010.
Bryant Haliday was one of the founders of the noted film distribution company Janus Films. He would later appear in another MST3K'ed film, The Projected Man. William Sylvester and Alan Gifford both later appeared in the critically acclaimed film 2001: A Space Odyssey (1968).
Frederick E. Smith wrote the original story for London Mystery Magazine in 1951, earning £10 for it. He said that one of the conditions of cashing his cheque was that he had surrendered any rights of resale of the story.
The script was originally written in 1957. The budget came from Gordon Films, Galaworld and the NFFC.
Sidney J. Furie was originally scheduled to direct but was offered a more prestigious film, so he recommended his fellow Canadian Lindsay Shonteff. Richard Gordon later said Furie advised Shonteff throughout the making of the film. Shonteff had to re-edit the film to avoid an X rating from the British Board of Film Censors.
Gordon later said the cost of the film was £20,000 plus $20,000 for expenses and salaries of American personnel, including Gordon and Bryant Halliday, making an estimated total of $60–75,000.