At the Griffith Theatre, magician Dante is about to return to the stage to take another bow when he is overcome by dizziness and later professes that he finds it difficult to distinguish between reality and illusion. That night, Dantes assistant for his guillotine act, Velma, quits and warns stage manager Gwendolyn Spears, who is secretly in love with Dante, that the magician is dangerously insane. In the lobby, while playboy and singer Beau Joe impatiently waits for Velma, with whom he has date, he jokes with theater owner Griffith that he wishes the guillotine would chop Velmas head off. The next day, after the newspapers report that Velmas decapitated body has been found, Inspector Evans interrogates members of the cast and crew. Joe admits to his conversation with Griffith and Gwen acknowledges that she fought with Velma over her quitting. Mama Rose tells the inspector that Joe once threatened her with a knife and that she believes he is the murderer. When questioned, Dante suddenly states that Velma had to go and collapses from one of his spells. Later, when Evans asks for an alibi, Dantes devoted assistant Carter claims that he and Dante were playing chess. That night, Dantes new assistant, Elinor Benson, tells her roommate Sharon that she is quitting because she fears Dante. After Elinor is discovered burned to death in her room, Evans arrests Sharon for the crime, but explains to Lt. Hennessey that the arrest is just a misdirection to bring out the real murderer. In the middle of the next guillotine act, Gwen, who has taken the victim role, suddenly realizes that when his assistants begin to fear them, Dante confirms their fear by killing them. As she screams for Evans help, Carter calls out maestro, maestro.