Adapted from the play by George S. Kaufman and Alexander Woolcott, an actor assumes a disguise to murder his sister's sinister husband.
Damon Welles (Edward G. Robinson) criticizes the play he is acting in and the acting of Daphne (Mae Clarke). His sister Jessica (Mary Astor) is making a come-back. Damon and Jessica appear to argue, but it turns out they are rehearsing a scene. Ben Weston (Ricardo Cortez) intends to produce the play in New York, and Damon agrees to do his part. When Stanley Vance (Louis Calhern) comes in, everyone else is silent. Jessica thought her husband was dead, but Vance is out of prison. Aunt Martha tells him to leave; but when he gets Jessica to go with him, Damon lets him stay. Dr. Kendall (Arthur Byron) says Jessica is possessed. Apparently she has been hypnotized to obey Vance. He gets her to visit Ben about the play. Vance kisses Daphne's hand; Damon comes in and tells Daphne to leave. Damon tells Vance he is ruining Jessica's acting, and he accuses him of living off women and killing his previous wife.
Vance tells Jessica he wants to get money for the play from a Frenchman named Chateau, and he has her tell Ben she can do the play. Ben tells Jessica he loves her. Ben and Vance fight, and Jessica gets someone to stop Ben from choking Vance. Chateau calls for Damon and talks with Vance, inviting him to meet at his hotel about financing the play. Vance brings Jessica to Chateau, who asks her to leave. Vance offers Chateau his wife, and they drink. Vance wants to sell his rights for $50,000. Vance is poisoned. Chateau puts his body in the closet, wipes off fingerprints, and leaves with his bags.
A jubilant Ben calls Jessica. Dr. Kendall says she collapsed. Chateau calls on Ben and removes his mustache and goatee to reveal himself as Damon. The police find no clues except a mustache in the Bible. The inspector clears Jessica and asks Damon to identify the body. The play opens, and Jessica wants to go to Capri with Ben. The police detective suspects an actor and calls on Damon, saying Vance killed his wife and was imprisoned for hypnotizing a woman. He saw Damon in a play as a Frenchman and shows him the mustache. He will give Damon a few days to find another actor to replace him. In the final scene Damon goes on stage.
Because Vance is portrayed as such a scoundrel, everyone's sympathies are to do away with him - except for the censors, who insisted that Damon not be allowed to get away with murder. Otherwise a skilled character actor might have been able to fool everyone if he hadn't left the mustache clue.