George "Babyface" Nelson becomes one of the most important gangsters of 1920s Chicago by making brutal robberies. In order to compete with Al Capone he allies himself with John Dillinger...

In Chicago during 1933, robber Lester M. Gillis is released from Joliet prison and plans to go straight. He is immediately coerced back into a life of crime, however, by gangster Lou Rocca, who has arranged for Gillis' release in order to employ him as a henchman. Although Gillis refuses his first assignment, the murder of a labor organizer, he accepts a job more to his liking, the roughing up of some policemen refusing to join Rocca's racket. Before starting work, Gillis visits his girl friend, Sue Nelson, to set up a date for later that night. Noting a newspaper story that the labor organizer has been killed, Gillis is pleased that he has avoided the job. As he awaits Sue in his apartment that evening, however, police barge in and, upon searching the toilet, find a gun taped to the underside of the tank. Rocca has framed him for the murder and Gillis is soon back in jail, where he formulates an escape plan. While he is being transported to another prison, he manages to beat up the guard and jump into Sue's waiting getaway car. Although Gillis warns Sue that he plans to kill Rocca, she refuses to leave him, despite the danger. They return to Chicago, where Gillis shoots Rocca and two henchmen in cold blood and then, exhilarated by his success, holds up a pharmacist to obtain illegal liquor. The pharmacist shoots back, however, and hits Gillis in the shoulder, forcing the couple to seek treatment from the seedy, alcoholic Doc Saunders. As Gillis recovers, he meets fellow patient John Dillinger, who has received plastic surgery to alter his features and fingerprints. Dillinger nicknames the young-looking Gillis, who has taken Sue's last name as an alias, "Baby Face." Soon, Baby Face is working with Dillinger's gang robbing banks, and his eagerness to murder anyone in his way earns him the second slot, after Dillinger, on the FBI's Most Wanted list. As federal agent Markie trails the gang, Baby Face chafes under Dillinger's leadership. When Dillinger orders him to hide out at a lodge, Baby Face complies but secretly sets up another robbery, with the help of Fatso Nagel. The agents have followed Nagel, however, and the gang barely manages to avoid arrest. During the escape, Baby Face insists on stopping the car to shoot the lodge proprietor and several agents. Their car later runs out of gas and Baby Face orders Sue to entice the driver of the next car into stopping to help them. When the kindly driver recognizes Baby Face, the gangster kills him, to Sue's horror. Soon after, Dillinger is murdered and Baby Face takes control of the gang, who cower at his ruthlessness. Now the number one Most Wanted criminal in America, Baby Face has his fingerprints removed by Saunders, but after the doctor botches the operation, the gangster kills him. Baby Face continues his bank-robbing spree, killing many hostages along the way but allowing one, a bank manager who happens to be of small stature, to live. Markie then arrests Nagel and forces him to set up a fake robbery in which the agents will be able to capture Baby Face. The gangster guesses the ploy, however, and after sneaking the gang into the bank disguised as a guards, locks his accomplices in the vault and takes off with the stolen money. He and Sue hide out at the lodge, assuming no one will look for them there. One day, two young boys wander onto the property, and Sue watches as Baby Face trains his gunsight on them. When the boys turn to leave, Baby Face lowers his gun, and later Sue asks him to lie to her that he would not have killed them. Soon, Markie learns about the hideout, and Baby Face and Sue once again flee. They are trapped by a roadblock, however, and although they try to crash through it, the agents shoot their car's tires, forcing the couple to run. Baby Face is shot multiple times, and struggles, with Sue's help, into a graveyard. There, he begs Sue to "finish him off," and when she refuses, tells her over and over that he would have shot the boys. Finally, Sue shoots her lover, then collapses in grief.