In a 1920s metropolis, when dance instructor Alice rejects charming petty thief Jack Diamond's invitation for a date, stating that she has entered a dance competition with Mr. La Tour, Jack injures La Tour's foot so that he might replace him. Unaware of Jack's malicious act, Alice accepts him as her partner, but the two win the trophy only after Jack surreptitiously sets their competitor's dress on fire. Later that evening, after he takes Alice to the movies, Jack exits through a bathroom window, robs a nearby jewelry store and quietly returns to his seat without Alice realizing the deception. When Lt. Moody interrogates theater patrons later, Jack uses Alice as his alibi, thus escaping further scrutiny, but is later sentenced to jail for the crime. When he is paroled, Jack asks Alice to give him a job as her dance partner, but upon completing his parole one year later, Jack abruptly quits and leaves Alice. Seeking to avoid another prison sentence, Jack tells his tubercular brother Eddie, also a petty thief, that they will start stealing from criminals who cannot call the police, and decides to become underworld czar Arnold Rothstein's bodyguard to gain access to the racket. After being refused an audience with the gangster and learning that Rothstein is Miami, Jack and Eddie steal from a small-time money launderer and use the money to send Jack to Miami. Charging over $4,000 worth of merchandise in Miami to Rothstein's account, Jack prompts the gangster to have his bodyguards drag Jack in to see him. Although Rothstein will not hire him, jovial gangster Little Augie does, but shortly after, the McDermitt brothers shoot both Augie and Jack. Although Augie dies, Jack miraculously survives, leading him to believe that "a bullet hasn't been made that can kill me." After mastering both a right- and left-handed draw and perfect aim from retired Sgt. Cassidy, a retired police officer, Jack easily kills the McDermitt brothers and then presents the murders as proof to Rothstein that Jack should be his bodyguard. Rothstein introduces him to crime syndicate bosses Leo Bremer, Waxey Gordon, Big Harry Weston and Frenchy La Marr, and hires him, giving him the nickname "Legs." Rothstein then sends his girl friend Monica to discern Jack's motivations. Inviting him to her apartment, Monica seductively suggests an affair, but Jack tells her to wait until he is wealthy and Rothstein is dead. After leaving Monica's apartment, Jack listens at the hotel switchboard as Monica calls Rothstein to report on his conduct. Instead of firing him, Rothstein promotes Jack to a collector and gives him a pocket watch as a token of his trust. Jack takes notes as he makes his collection rounds from prostitutes, bootleggers and the gambling ring at the Hotsy Totsy nightclub. In order to learn how Rothstein's smuggles narcotics into the country, Jack has sex with Monica, who gratefully divulges the underworld czar's secrets. When Rothstein questions them about having an affair, Mon