After a mental patient escapes, Chan solves two murders that occur during an opera.
An amnesiac Gravelle (Boris Karloff) sings opera and escapes from a sanitarium in order to find singer Lilli Rochelle. Police search for him. Chan says, "Politeness, golden key that open many doors." Lilli Rochelle (Margaret Irving) brings a threatening note to police Inspector Regan and Sergeant Kelly (William Demarest). Anna Borelli (Nedda Harrigan) is jealous of Lilli. Agnes sees Gravelle hiding behind costumes. Lee Chan (Keye Luke) is disguised in armor. Kelly has Childers (Thomas Beck) and Kitty (Charlotte Henry) thrown out. Whitely (Frank Conroy) warns Enrico Borelli (Gregory Gaye) to stay away from his wife Lilli. Gravelle tells Anna Borelli that he will sing tonight. Chan asks Anna about the flowers that went from her room to Lilli with the threat. Lee Chan gives his father stolen items with finger-prints. Whitely threatens to divorce Lilli. Gravelle tells Enrico that he is replacing him and says he was locked in a burning theater.
On stage Gravelle in a mask sings with Lilli and stabs her. Gravelle knocks down Kelly and runs away. Chan says that Enrico was stabbed earlier. Kelly finds Childers in Lilli's room. Lilli is dead. Whitely takes a knife from the flowers, ruining finger-prints. Chan questions Whitely and Childers. Kitty says that she is Lilli's daughter and that her father died in a theater fire. Gravelle has changed clothes and listens from above. Lee searches and finds Gravelle's costume. Chan has Lee call the newspaper to get a photo of Gravelle. Agnes identifies it as the man she saw. Childers re-assures Kitty and goes out to be questioned. Gravelle comes in and plays piano for Kitty, asking if she remembers. Kitty faints. Chan hears the music and finds her. Gravelle tears out the phone line and picks up a knife. Chan reminds Gravelle his world is music and asks him to sing. Chan persuades Kelly to let him and Anna sing Lilli's part.
Gravelle sings, draws his knife, and is shot by police. Chan says that Gravelle's knife was not used in murder. Chan asks Whitely why his finger-prints are on the threatening note, and Whitely explains. Chan says that Anna Borelli murdered her husband and Lilli. Anna confesses and says Whitely was not man enough to do it. Gravelle will recover, and Chan sends Kitty to him. Chad explains to Kelly that he had Anna sing to loosen her tongue.
Once again the Chinese detective solves the mystery with style and perception while humbly saying he is lucky. His gentility and common sense offer a positive role model in this escape entertainment.