During the search for bodies among the wreckage of a boat lost in a storm near Hawaii, Charlie Chan finds a diary belonging to Allen Colby, heir to his father’s fortune that has been brought up from the sunken ship. The diary offers proof that Colby had been aboard the vessel, and what is written inside the book reveals that attempts had recently been made on his life.
Chan sends a cable to Henrietta Lowell, matriarch of the family, an acquaintance of the detective and the sister of Allen’s deceased father. In his message, Chan states that Allen Colby’s fate is uncertain. He flies to San Francisco where Mrs. Lowell, a devoted believer in “psychic research,” schedules a séance.
Allen Colby secretly returns to the estate, but, upon entering, he is hit by a thrown dagger.
Later, Chan arrives at the Colby Estate. He has found that, seven years before, Allen had joined the French Foreign Legion. During hostilities, Colby had been captured by Rifs and was held prisoner, but had eventually escaped. All of that time, he had been out of contact with his family.
That evening, the séance is conducted by Carlotta, a medium, and is attended by Professor Bowan, whom Mrs. Lowell supports financially. Also at the séance are Mrs. Lowell’s two daughters, Alice and Janice; Alice’s boyfriend, newspaper reporter Dick Williams; Janice’s husband, Fred Gage; Ulrich, a disgruntled caretaker, who blames his daughter’s suicide seven years earlier on Allen Colby, who had broken his engagement with her; and Chan. During the séance, Allen Colby, whose face is strangely illuminated, appears on the stairway leading to the darkened room. The figure of Colby suddenly falls, and, when the lights are turned back on, Chan discovers that Allen has been murdered.
Chan, hoping to begin his investigation of Allen Colby’s murder, arranges with Mrs. Lowell to allow him to spend the night in the unoccupied Colby House, a mysterious place that has many secret panels and passages. Mrs. Lowell leaves the detective to do his work, but, curious herself about the murder and its connection with the séance, she returns to join Chan in his search for clues.
While exploring the room where the séance was held, Chan finds a radio receiver that, he determines, probably accounts for the eerie music that was heard during that evening’s séance. Chan also finds that Allen’s face had appeared to glow in the darkness because of a hidden ultraviolet light that had been projected toward it. Allen’s face, Chan finds, had been covered with quinine sulfate, a solution that glows under ultraviolet light.
Greatly upset over what has been discovered, Mrs. Lowell reveals that Professor Bowan knew that if Allen were to return, her patronage of his “research” would come to an end. As she agrees with Chan’s suggestion that she immediately revise her will, now that she is the legal heir, a shot is fired at her from a hidden doorway. However, Chan had seen the reflection of the assailant’s gun in a mirror, and, acting quickly, had pushed Mrs. Lowell out of harm’s way. Just then, Gage enters the house, saying that he had become concerned for his mother-in-law’s safety.
The next day, after seeing a radio transmitter in Ulrich’s cottage on the grounds of Colby House, Chan visits Carlotta. She admits to the use of the transmitter, but she is shocked to learn of the other deceptive methods that were employed by Professor Bowan. As Chan inspects the coil of the short wave transmitter at Carlotta and Professor Bowan’s residence, Bowan, who is hidden, switches on the current. Chan receives a strong electric shock and passes out. Bowan tries to convince Carlotta to flee with him, but she refuses. Just then, the police arrive, but Bowan escapes. Inspector Morton arrests Carlotta as Chan is revived.
Later that day, Mrs. Lowell announces to her family that she plans to sign a new will, which her executor, Warren T. Phelps, will bring to her home at 8:30 that evening. As Mrs. Lowell sits at a table near a window with Baxter, her butler, the bell in a nearby tower signals 8:30. At this moment a bullet smashes through the window, hitting her in the head.
Chan later investigates the bell tower and finds a high powered rifle, which has been set to fire as the bell sounds at the spot where Mrs. Lowell had customarily sat each evening at 8:30.
Bowan is caught, but he does not confess, and Chan calls all of those who were present at the previous séance back as Carlotta, with Professor Bowan, conduct another. During the séance, Mrs. Lowell’s animated body appears. As it begins to speak, saying that the murderer will be revealed, a knife is suddenly thrown at the image, which breaks, as the image had been but a reflection in a the mirror. When the lights are turned on, Chan reveals that Mrs. Lowell is alive, and that it was a dummy that had been shot earlier. Gage accuses Ulrich, and, as they begin to struggle, Chan grabs Gage’s hand and opens it, revealing dark smudges of graphite that the detective had placed on the knife, the twin of the one that had been used to murder Allen Colby. The detective accuses Gage of murdering Colby to cover evidence of forgery in the Lowell accounts.
After Gage is taken away, Mrs. Lowell refuses to press charges against Carlotta and Professor Bowan. However, Inspector Morton “suggests” that the couple leave town, and they agree to do so.