Julius Ricardo is holidaying on the French Riviera with his friend, the famous detective Hanaud. Visiting the casino, Ricardo is troubled by the sight of a strangely familiar young woman. She departs in the company of a rich older woman, Madame Dauvray, who lives at the Villa Rose. Ricardo recognises an old acquaintance, Harry Wethermill, who tells him that he is to marry Celia Harland, Dauvray's young companion.

The next morning, Wethermill rushes into Ricardo's hotel room, announcing that there has been a murder at the Villa Rose: Madame Dauvray is dead and Celia is missing. Wethermill implores Ricardo to enlist Inspector Hanuad to help find Celia and clear her name, as she is now the chief suspect. Hanaud agrees to take the case, and the three men are soon on their way to the Villa Rose. Perrichet, the French policeman who discovered the crime, recounts how he passed the villa earlier in the evening and saw that the gate was open. Finding it still open later that night, he went to investigate. In the salon he found Madame Dauvray's dead body and upstairs discovered Helene, the maid, unconscious. The chauffeur tells Hanuad that Madame Dauvray's car, with enough petrol for a 200-mile trip, is missing. He admits that Celia gave him the previous night off. The detective discovers a number of clues in the salon before finding Celia's compromising footprint in the gravel outside the house. A policeman arrives and tells Hanaud that the cord used to strangle Dauvray was undoubtedly purchased by Celia.

With all the evidence arrayed against Celia, Helene gives her testimony. She tells the assembled men that she hated Celia, but admits to helping her perform fake séances for the highly superstitious Madame Dauvray to prevent her from being exploited by unscrupulous fortune-tellers. Ricardo recalls that he once saw Celia giving a spiritualist performance in England. When Helene has finished her story, Hanuad tells her that she is free to go. The men examine Madame Dauvray's bedroom. From its ransacked state, Hanaud deduces that the woman's jewellery cannot have been in her safe, which is standing open and empty. He finds a hidden compartment under the floorboards and retrieves the jewels, much to the astonishment of those gathered. Hanaud tells them that nobody must hear of this discovery. Later, Madame Dauvray's car is found and Perrichet realises that he saw it driving past the Villa Rose early that morning.

The following morning, Ricardo is disturbed yet again, this time by Hanaud, who tells him that a woman has answered an advert that he inserted in a Marseilles newspaper asking for information on Celia. The witness is due to arrive later that afternoon, but she is murdered on her way to the hotel. Hanaud summons Wethermill to Ricardo's room and tells him that he knows that Helene was an accomplice to the murder. He accuses Wethermill of murdering both the witness and Madame Dauvray. Wethermill tries to escape but is restrained by several policemen who are waiting outside the door. Wethermill confesses to both crimes and relates how he, Helene, and another woman, Adele conspired to steal Madame Dauvray's jewellery and throw suspicion on Celia.

Adele persuaded Madame Dauvray to invite her to a séance. By pretending to be highly sceptical, she was able to tie Celia up herself. During the séance, Wethermill crept into the house and strangled Dauvray. The conspirators found the safe but it was empty. They tried to get Celia to tell them where the jewels were hidden but she claimed not to know. Celia was forced to leave her footprint in the gravel outside the villa, then Wethermill drove her and Adele back to Marseilles. Celia was kept alive in the hope that she would eventually reveal the whereabouts of the jewels.

As Wethermill finishes his confession, a policeman bursts in and tells Hanuad that someone from Adele's house has bought the newspaper reporting the finding of Dauvray's jewels. Celia's life is now in danger and Hanaud and Ricardo rush to Marseilles to save her. They arrive in the nick of time and Adele, the last of the conspirators, is arrested.