At the Rockland State Sanitorium, Gravelle, an opera singing amnesiac, regains his memory when he sees a newspaper article about prima donna Lilli Rochelle, then kills a guard to escape. Inspector Regan calls Charlie Chan in on the case, and as they are in his office discussing it, Lilli comes in, accompanied by her lover and fellow singer, Enrico Barelli, to complain about a threat stating she will die that night. Chan agrees to go to the opera that night, along with Sergeant Kelly, to investigate. Later at the theater, Phil Childers and his girl friend Kitty try to see Lilli but are turned away by Kelly just as Regan and Chan arrive and hear Lilli's husband Whitely and Enrico fighting over Lilli. Meanwhile, in the dressing room of Enrico's wife Anita, Gravelle appears, and although Anita is terrified because he was presumed dead in a theater fire years ago, she agrees to keep his presence a secret while he carries out his plan of singing Enrico's role on stage. Gravelle then menaces Enrico, who, along with Lilli, locked Gravelle in the burning theater, and soon it is Gravelle rather than Enrico who joins Lilli on stage for their duet. Lilli recognizes Gravelle's voice and faints after she leaves the stage. After Whitely carries Lilli off, the others rush to Enrico's room, only to find that he has been stabbed. While the others search for Gravelle, Phil enters Lilli's room and discovers that she is also dead. Whitely comes in and has Phil arrested, but when Chan questions Phil and Kitty, they tell him that Kitty is Lilli's daughter from her previous marriage to Gravelle and that Lilli refused to acknowledge Kitty in order to keep her past a secret. The young lovers were there to ask Lilli for her permission to marry, as Kitty is underage. Gravelle, who did not recognize Kitty, is stunned as he overhears. Later, Phil goes to see Regan, leaving Kitty alone, and Gravelle comes in. He gently questions her and plays the piano for her, but she does not remember him and faints from fright. Chan enters, and after Gravelle tells him about Lilli and Enrico's attempt to kill him, Chan flatters him into singing again. Chan arranges to have Anita sing Lilli's role, and during the duet, which involves Gravelle's character stabbing Anita's character, Anita becomes so scared that a police officer shoots Gravelle. Chan then demonstrates that Gravelle's knife is a prop and could not have been used in the murders. He explains that Anita was the only one who had access to Enrico and Lilli when they were alone and unconscious, and that she was also the only one who knew Gravelle was there and could therefore frame him. Anita confesses that jealousy drove her to kill her husband and his lover, and after she is taken away, Chan convinces Kitty to comfort injured Gravelle, thereby saving his life.
