Michael Morgan, a manufacturer of magician's tricks, meets Judy Barclay when she runs into his shop to escape a man who is following her. Judy asks Mike's help in debunking the psychic Madame Rapport, but before divulging any further information, she is silenced by a threatening glare from Dr. Sabbatt, an expert on demonology who is browsing in Mike's shop. His interest piqued, Mike follows Judy to Sabbatt's apartment and watches as she runs terrified from the building. Returning to the apartment later that night, Mike finds the door chained from the inside and Sabbatt murdered, his corpse circumscribed by a devil's circle. Puzzled, Mike sends for Inspector Gavigan of the police department, who questions Madame Rapport, Mrs. La Claire, Tauro and Dave Duvallo, all psychics and magicians who had visited Sabbatt around the time of his murder. Tauro, who had been blackmailing Sabbatt, is the Inspector's prime suspect until Mike finds his corpse and the coroner announces that Tauro was murdered before Sabbatt. The mystery deepens when Mike next finds mysterious messages claiming that Madame Rapport is Sabbatt's wife and Judy's sister. When Mike questions Judy, she admits the relationship and explains that Sabbatt was rigging tricks so that Madame Rapport would win the $25,000 psychic society award. In an attempt to trap the killer, Mike assembles the suspects for a seance in which a projected image of Sabbatt states that Judy has a clue to the identity of the murderer. Using Judy as bait, Mike tricks the killer into trying to murder her during a sharp shooting demonstration, and when the disguised figure flees from the fake shooting, Mike catches him and unmasks Duvallo.