An opera singer is murdered during a performance; a detective and his girl-friend discover the surprising killer.
At the Hollywood Bowl a swami (Pedro de Cordoba) tells Gino D'Acosta (Leo Carrillo) if he sings in the opera, he'll die. Gino kisses Diana (Benita Hume). Then Louisa (Katharine Alexander) complains to Gino and kisses him. The composer Bejac (J. Carrol Naish) wants Gino to sing Don Juan in his opera. Diana tells William (Frank McHugh) to call Dr. Adams (Grant Mitchell) to stop Gino from singing. Gino is stabbed, but Dr. Adams says he is all right, while Toni Adams (Madge Evans) treats his cut. Detective Steve Farrell (Chester Morris) questions Bejac. Dr. Adams pronounces Bejac insane, and police take him away. Gino gets a threatening doll with a pin in it and pretends his heart hurts. Police chief Quinlan (Robert McWade) orders Steve to take Bejac to the asylum.
During Verdi's Il Trovatore Gino tells Louisa not to be jealous but gives her bracelet to Diana. Louisa comes in while Gino is kissing Diana. Bejac escapes by knocking out an officer and pushing Steve out of the car. William sings into a microphone, and the audience laughs. Steve arrives, and Toni treats his hand. Gino dies and is replaced by Ivan Bosloff. Doctors said Gino died of a heart attack; but Toni finds poison in wine, though no poison is found in Gino's stomach. Bejac says he did not kill Gino, and witnesses back him up. Steve finds Toni in his bed. Toni brought Louisa, who says Gino's letters must not be exposed. Steve goes and gets them, planning to read them. Diana opens a jar, screams, and faints. Dr. Adams says she died the same way as Gino. Pedro (Duncan Renaldo) had a gun there and is arrested. Steve finds broken glass and discovers the letters are gone.
Steve goes to Toni, who burned the letters. Toni finds poison on the glass. Toni tells Quinlan she burned the letters, and Steve gets demoted. Steve and Toni find Dr. Adams passed out after someone sprang upon him with a vapor. Steve tells Toni that poison was put in the microphone and that it was in the test tube but not in the wine. Dr. Adams admits he killed Gino to spare him a painful death. He explains that Diana died by mistake and that he knocked himself out to clear Pedro. Bejac comes in with a gun, shoots at Dr. Adams, and is killed by police. Steve confirms Quinlan's suspicion that Bejac confessed and then quits; but Dr. Adams turns himself in to Quinlan, and Steve kisses Toni.
Some fine opera music and occasional comedy enhance this clever mystery story. The audience is kept guessing until the end, when the killer's motive appears honorable, although Louisa's death is his fault too.