While holding a seánce at his home, psychic W. Bonner asks for questions from those present. After a voice asks what happened in London on 5 Oct 1935, the lights are extinguished, a gunshot rings out and Bonner is found murdered. As Bonner's wife Justine summons Inspector Matthews of the homicide bureau, Bonner's assistants, Tom and Vera Starkey, who operate from a hidden chamber, panic and flee. Matthews orders all those present at the Bonner house to headquarters for questioning. Among the suspects are seánce particpants Harriett Green, Paul Hamlin, Charles Edwards, Nancy Wood, newly hired valet Birmingham Brown and Frances Chan, the daughter of famed Honolulu detective Charlie Chan, who had been alerted to the seánce by her old friend Brown. When the coroner notifies Mathews that the bullet has disappeared in Bonner's body, the inspector coerces Chan into joining the investigation. After studying the interviews with the suspects, Chan asks to speak to Nancy Wood and informs the inspector that she must be using an alias because the monogram on her purse reads "ND". A conversation with the hotel clerk reveals that Nancy's real name is Norma Duncan. Confronted with her lie, Norma claims that she used the alias to infiltrate Bonner's seánce and prove that the psychic drove her father to commit suicide. At the Bonner house, meanwhile, Mrs. Bonner communicates to Tom and Vera through an intercom linked to their hidden chamber. When Frances, who has come to visit Brown, overhears the conversation, she alerts her father. Hurrying to the house, Chan discovers the secret chamber hidden behind a cabinet in the seánce room. After reassembling the suspects at police headquarters, Chan reveals that Mr. Hamilton and Mrs. Green were both being blackmailed by Bonner and that Mrs. Bonner had threatened to kill her husband because of his infidelity. Chan then asks if anyone can identify the speaker at the seánce who framed the question about London. Later, Norma sends Chan a message to come to her hotel, but when Chan enters her room, he finds her in a trance. Just then, the lights go out, a gun is fired and the assailant runs out the door. Awakened from her trance, Norma is unable to remember what happened to her, and Chan entrusts her to Rafferty, Mathew's assistant, while he goes to send a cable to Scotland Yard. Later, at the Bonner house, Frances and Birmingham overhear Mrs. Bonner asking the unseen Tom and Vera for their help, and Frances decides to follow her. Soon after, Norma phones the inspector to inform him that Edwards is acting strangely on the second floor of the Berekley building. Following Mrs. Bonner to the Berekely building, Frances meets her father and the inspector there and finds Mrs. Bonner's glove outside a closed door. Mrs. Bonner, in a trance, ascends to the rooftop where a voice coaxes her to jump from the ledge. After she plunges to her death, Chan notices a peculiar stain on her coat and takes it to the lab for analysis. The coroner identifies it as the residue of a powerful mind-altering drug that renders its victims helpless. He then gives Chan several pills that contain the antidote to the drug. Announcing that Mrs. Bonner committed suicide out of remorse for murdering her husband, the inspector declares the case closed. Dissatisfied with the inspector's conclusion, Chan returns to search the room in front of which Mrs. Bonner's glove was found. There Chan is manacled and hypnotized by an unseen assailant who orders him to climb to the roof and jump. After swallowing one of the pills, Chan proceeds to the roof. Meanwhile, becoming concerned when she is unable to locate her father, Frances notifies Rafferty. Together they hurry to the Berekley building and trace Chan to the roof. As Chan totters precariously on the ledge, the antidote begins to work and he comes out of his trance just as Frances and Rafferty appear. When Rafferty hands Chan the response from Scotland Yard, Chan reconvenes a seánce at the Bonner house. That night, Chan exposes Tom and Vera in their secret chamber and then challenges the seánce participants to answer a series of word association questions. After the lights go out and another gunshot is fired, Chan produces a cable from Scotland Yard imparting the information that on 5 Oct 1935, Chardo, a famous magician, was gravely injured in a car accident. Chan continues that Chardo's wife, Justine, had just run away with his assistant, Bonner. After undergoing plastic surgery to alter his appearance, Chardo determined to make Justince and Bonner pay for their betrayal. Chan then extracts a bullet made from frozen blood from his chair, thus explaining the disappearing bullet that killed Bonner. Addressing Hamlin as Chardo, Chan seizes the cigar case in which is hidden the spring gun used to kill Bonner.