When three murders occur over stolen bonds, Chan solves the case.
Monte Carlo Police Inspector Joubert (Harold Huber) pays for the room of Charlie Chan (Warner Oland) and his son Lee Chan (Keye Luke). Gordon Chase (Robert Kent) tells Joan Karnoff (Kay Linaker) to bring him bonds worth $25,000 that are missing. Joan tells bartender Al Rogers (George Lynn) that she needs the bonds, and he asks for cash. Joan hands the bonds to Gordon before her husband Victor Karnoff (Sidney Blackmer) can learn they are gone. Chan and Lee have car trouble and walk. They see a sports car drive off and find a man shot in a car. Police arrest them when Lee's French fails. Joubert releases them, and Chan gives him a rhinestone they found. Joubert calls Victor about his car. Victor says the dead messenger was carrying a million in bonds. They note that bartender Al tried to sell $25,000 in bonds. They see the sports car and call on Evelyn Gray (Virginia Field), finding Paul Savarin (Edward Raquello), Victor's competitor. From evidence Chan suspects they were at the scene. Evelyn says she saw the dead man. Victor and Gordon come in, and Victor accuses Paul. Victor admits the bonds are insured. Rogers arrives, but a fire in the trash burns the bond records. Joubert learns that the chauffeur Ludwig was found dead near the crime scene.
Rogers calls Joan and demands money. Evelyn promises Gordon that she won't see Paul but then goes to Paul to say they can't get information from Gordon anymore. Lee tries to listen. Joubert learns that Joan pawned jewelry for $25,000, and Chan says, "Questions are keys to door of truth." Lee is arrested for peeping in a window but runs off. Joubert and Chan find Rogers dead in his room and the bonds in a bag. Joubert concludes suicide; Chan says no, but the thief was there. Joubert and Chan find Evelyn and Paul at Victor's house. Joan admits that Rogers was blackmailing her because they were married and not divorced. Gordon comes in and says he saw the dead man and canceled his flight. Gordon was with Evelyn in her car. Joubert asks Evelyn how she got an expensive car. Chan says that $200,000 in bonds were not in the bag and are in Victor's safe. Gordon has the key to the bag, and Chan says he is the murderer and that he bought things for Evelyn. Gordon confesses, jumps out the window, and is run over by a car. Joubert tells Evelyn and Paul to leave Monte Carlo and then says good-bye to Chan and Lee.
Spiced by Lee's faulty French and Joubert's amusing English, this mystery unfolds with only occasional help from the clever Chan, as blackmail and a corrupt romance forward the plot.