Chan is on a gambling vacation in Monaco. He is called upon to solve two murders. One is a casino messenger on his way to Paris with a million dollars in bonds. The other is a two-bit Chicago gangster recently tending bar in a Monte Carlo hotel. Everyone is suspect and a third of the dialogue is in French.

Charlie Chan and his son Lee attempt to leave Monte Carlo, where Chan is a shareholder in a casino, to go to Paris, where a painting by Lee will be exhibited, but on the road to the Nice airport, their taxi breaks down. Chan and Lee continue on foot until they see an expensive roadster leave another car and find a dead man in the other car. Upon investigating, they learn that the dead man was Renault, a bank messenger on his way to Paris with $1,000,000 in metallurgical bonds, now missing, which belonged to Victor Karnoff, who is staying at the Hotel Imperial. They also discover that three metallurical bonds were that day offered for sale at a Monte Carlo bank by Al Rogers, a bartender at the hotel. Chan recognizes the roadster in front of the hotel and learns that it belongs to Evelyn Grey, who is a resident there. Police inspector Joubert and Chan find her with Paul Savarin, a stock broker and Karnoff's bitter rival. Although she admits stopping by the car, Evelyn says that she fled in fright when she saw Chan and Lee approach. Joubert learns from a messenger that Karnoff's chauffeur Ludwig, who drove Renault and was in league with Savarin, was found dead. Later that day, Karnoff's wife Joan, who earlier begged Rogers to give her back the three bonds, admits to him that she stole them from his room, and he gives her until noon the next day to repay him. The next morning, Evelyn promises Gordon Chase, Karnoff's secretary, that she will not see Savarin again. After Chan learns that Evelyn has been living in luxury with no visible means of income and that Joan pawned jewelry that morning for $25,000, the price of the three bonds, he and Joubert go to interrogate Rogers, but find him dead in his room with an open valise full of Karnoff's bonds. Joubert believes that Rogers committed suicide and that the case is closed, but Chan suggests they go to Karnoff's suite, where they also find Joan, Savarin, Gordon and Evelyn. Joan confesses that Rogers, whom she married seven years earlier and whom she thought had obtained a divorce, was blackmailing her. She had given him the three bonds, but stole them back when she learned of her husband's deal, and had her brother, Gordon, replace them in Karnoff's safe. Chan surmises that Gordon must be the murderer because only he had a key to the valise, and that the valise was found open in Rogers' room, with no damage to its lock. Gordon confesses and berates Evelyn, for whom he says he stole from Karnoff months ago. When he was in danger of being exposed, he murdered Rogers to place the blame on him. Gordon then escapes through a window, but he is run over by a car. Joubert insists that Evelyn and Savarin leave Monte Carlo and gives his friends, Chan and Lee, another send-off in the same, backfiring taxi.