Michael Shayne is a private detective who is disliked greatly by Pete Rafferty, local chief of police detectives. Rafferty notifies the newspaper press that he is going to close Shayne's agency, just as Michael is about to be hired by the wealthy Eleanor Ramsey, who is being blackmailed. She is the stepmother of what she considers to be two grown-up brats, Dorothy and Ernest, and she considers their father to be of little value to the world himself. They all conspire to get their hands on her money, even to the extent of attempting to hire Shayne to frame an insurance robbery. Mrs. Ramsey is murdered, and Rafferty is trying to pin the killing on Shayne, despite the fact that suspicion points to Buell Renslow, brother of the slain woman. Shayne's secretary, the fetching Phyllis Hamilton, decides to do a little detective work to help her boss.

In the gay '90s, cardsharps take over a Mississippi riverboat from a kindly captain. Their first act is to change the showboat into a floating gambling house. A ham actor and his bumbling sidekick try to devise a way to help the captain regain ownership of the vessel.