Honolulu detective Charlie Chan arrives in Paris ostensibly on vacation after solving a noteworthy case in London. He makes an appointment to meet Nardi, a dancer at the Café du Singe Bleu, after her performance that night. Chan first visits Victor Descartes, whose father, Chan's friend, is a director of the Lamartine Bank where Victor works as a clerk. After Victor's fiancée, Yvette Lamartine, the bank president's daughter, arrives with two friends, Max Corday, an intoxicated sketch artist, and Renee Jacquard, the group accompanies Chan to the café. As Max gets out of his car, he bumps into a disgruntled man on crutches in dark glasses, who berates him. After her Apache dance, Nardi is murdered in a back room by a knife thrown by the man on crutches. In Nardi's room, Chan finds a diary containing information about Albert Dufresne. As Chan is leaving the building, the man on crutches drops a cement block from the roof, which nearly hits him. Chan is pleasantly surprised to find in his hotel room his son Lee, who has come to vacation with his father. Chan reveals to Lee that he is really investigating a case for a London banking house and that his accomplice, Nardi, has been murdered. The next day, when Yvette visits her father at the bank, Dufresne, her father's assistant, threatens to show love letters she once wrote him to Victor unless she visits his apartment. While Lee waits at the door, Chan enters the bank and witnesses the office manager, Henri Latouche, have the man on crutches, whom he identifies as Marcel Xavier, a shell-shocked, crippled soldier, escorted out for causing a disturbance. Chan meets with Lamartine and Dufresne and after showing them that bonds issued by the bank are, in fact, forgeries, he instructs Lee to follow Dufresne. That night, as Lee watches from the street, Yvette visits Dufresne, who is packing and is secretly being watched by Xavier. Just as Dufresne is handing Yvette the letters, he is shot from the room that Xavier entered. Xavier escapes with the bonds Dufresne packed, but Lee follows his taxi. Yvette, who grabs the gun when the room is invaded by people, is arrested for murder. When Max and Renee are interrogated by the police, Yvette slips the letters to Chan, who promises to destroy them. Lee returns to the hotel, and when Chan joins him, Lee reports that Xavier got into a limousine after the taxi ride and that it was the same limousine in which Max and Renee drove away from the hotel. Chan visits Max and tells him that he suspects Xavier used his limousine to get rid of his disguise. Max, thinking that Chan may suspect him, reminds him that Xavier bumped into him outside the café. Chan then leaves and Max packs the bonds that were in Dufresne's room, but Chan and Lee stop him from leaving. Chan surmises that Max killed Dufresne, but he still has not found Nardi's murderer. While Lee holds Max at gunpoint, Chan goes to the bank where Latouche gives him Xavier's address. Victor, appealing to Chan to help Yvette, drives him to find Xavier. At the address, Chan finds a secret panel leading to the Paris sewers, where they find a room with printing and engraving equipment and more forged bonds. Xavier arrives and fires at Chan, but they apprehend him and Chan removes his wig, glasses and mask to reveal Latouche. When the police arrive with Lee, Chan explains that Max and Latouche both used the disguise so that they each could have alibis. Dufresne, their accomplice in the bond forgeries, tried to leave town with their money and was murdered by Max. Chan then tells police inspector Renard that Yvette is his assistant and was sent by him to get important letters from Dufresne. Renard understands and agrees to release her.