Lingerie salesman Johnny Dill, thought to be a killer gangster, finds himself struck between the forces of good and evil.
In 1931, timid lingerie salesman Johnny Dill becomes depressed when he loses his girl friend, Judy Parker, to his smooth-talking childhood friend, lawyer William T. Allen. Instead of proposing to Judy, as he had planned, Johnny takes Millie Gardner, his assistant, to a movie. To Johnny's dismay, Millie is enthralled by the film's aggressive hero, gangster Big Nick Moronie. Fed up with his "nice guy" image, Johnny assumes a tough gangster demeanor and saunters into a speakeasy. There Johnny flirts with a gangster-crazy socialite and angers the real Big Nick Moronie, "public enemy number 21," when he pulls the mobster's nose in a foolish show of bravado. Later, Big Nick sends one of his thugs, the enormous Little Joe, to intimidate Maboose, "public enemy number 24." Instead, Maboose, who wants to move up in rank, bribes Little Joe to kill Big Nick, and the thug does the job in Big Nick's apartment, which is across the hall from Johnny's. To avoid detection, Little Joe then deposits Big Nick's body in one of Johnny's lingerie trunks. After he discovers the corpse, a terrified Johnny loads the trunk onto the back of a rental truck, but loses it during a high-speed chase with a policeman. Six days later, Johnny, who has been dubbed "Killer" Dill in the press, is hiding out when Judy convinces him to give himself up. At his trial, Johnny is defended by the publicity-hungry Will, and despite Will's blatant incompetency, he is acquitted. Although Johnny wins his freedom, he is still perceived as Big Nick's killer and is awarded the title of "public enemy number 21." As Big Nick's brother Louie has sworn revenge on Johnny, Will convinces his friend to team up with Maboose, who wants to exploit Johnny's newfound reputation as part of a plan to take over Louie's territory. The scheming Little Joe, meanwhile, arranges with Maboose to double-cross the unsuspecting Louie. Before Little Joe carries out his plan, however, Johnny, who has heard that Judy is about to marry Will and is finally going to confront the lawyer, waves a toy pistol at him. Believing that the toy is real, Little Joe admits to killing Big Nick, and Johnny then forces the thug to write a confession. When Little Joe finally realizes that the gun is a toy, he starts to strangle Johnny, but is stopped by an enraged Louie, who has overheard Little Joe's declaration of guilt. After Louie and Johnny shove Little Joe out the window, Will selfishly persuades Johnny to tear up the confession, claiming that it would hurt Judy because it would embarrass his boss, Maboose. Finally seeing Will for what he is, Judy breaks their engagement and rushes to propose to the ever-faithful Johnny.