A blackmailing murderer is caught by two nitwits trying to clear a girl-friend in this mysterious comedy.
The song "You Opened My Eyes" introduces the music publishing company run by Lake (Hale Hamilton). Newton (Robert Woolsey) has invented a machine that makes a person tell the truth, and Johnny (Bert Wheeler) says he loves Mary (Betty Grable). Mrs. Alice Lake (Evelyn Brent) shows her husband another letter from the black widow extorting money from them. Mary tells Johnny to write a song about murder. Newton catches Sleepy (Willie Best) cheating in a dice game. Darrell (Fred Keating) agrees to guard Lake. Mary wins a revolver from Johnny's game. Newton has a song he sings to Lake about how the black widow is going to get him. George Clark (Erik Rhodes) accuses Lake of cheating him on his song payments. Newton reads a torn note and thinks that Johnny is going to murder Lake with the revolver. Johnny slaps Lake and tells Mary to quit. Lake is shot from a hole in the ceiling. Newton finds him and runs out, and Johnny tells Newton he did it. Police arrive, and Riley (Edgar Dearing) guards Newton and Johnny. Mary picks up the gun and goes out. Newton tries to help Johnny escape, but he keeps coming back. Darrell finds Mary with the gun. Newton slips out of the handcuffs and shows Riley how to do it, leaving him in them. Johnny confesses to try to keep Mary from being arrested.
Newton and Johnny put on stilts to visit Mary in jail but are cut down to size. Alice Lake tells Darrell she got another threatening note, and Darrell says pay. Several people were told to put money in the Lake building, and the police stake it out. Alice Lake puts money in the ventilator. Sleepy lets blacks shoot dice upstairs. Newton sets up his truth machine in Lurch's chair. Newton and Johnny chase a songwriter and run into Darrell, who has them re-enact the crime. In the machine Darrell says he is the black widow. Newton sits in Lake's chair, but Johnny interrupts his being murdered. A man with a gun sits there and is shot from above. Newton and Johnny tell Darrell that the murderer is upstairs; but Darrell clubs both of them. Darrell stows money in the ventilator, and Sleepy uses it in the dice game. Mary and the police find the auditor Lurch dead in Lake's chair, and Clark is tied up. Newton and Johnny wake up and are chased by Darrell in a skeleton suit, which scares away the blacks. Newton and Johnny pick up money. Darrell and his men chase them, but Newton drops jugs on each of them. The police catch Darrell, and Mary and Johnny plan to marry and have children.
Wheeler and Woolsey manage to entertain by making mirthful madness out of a murder mystery.