Although unsuccessful in their first attempt to assassinate Chicago's Mayor, Anton J. Cermak, the Capone mob under the command of Frank Nitti and several other of the imprisoned mobster's lieutenants, have not given up. This time they hire a professional, Fred 'Caddy' Croner, an expert at using a long rifle with a scope who carries his weapon in a golf club bag. 

Just as a prisoner is about to turn stool pigeon, he is shot by someone on the inside. So Ness transfers a con from another prison to find the killer.