The city of Chicago led the nation when it came to gangland violence during the Prohibition era. As a result, many infamous, unforgettable personalities became a part of America's criminal history.

Chicago Assassin is the story of "Machine Gun" Jack McGurn, one of the people responsible for putting much of the roar into the Roaring Twenties. His family immigrated to Chicago from Sicily in 1906, as he grew up in the city's slums and later took up boxing as "Battling" Jack McGurn.

After avenging his father's death by killing the three hit men responsible, he came to the attention of Al Capone, who invited him into his organization, known as the Chicago Outfit. There he rose to power and was one of the most feared members Capone's organizations, with more than twenty-five known kills for the mob. "Battling" Jack McGurn became so adept with the Thompson submachine gun that he quickly became known as "Machine Gun" Jack McGurn.