Italian mob leader Big Louie Costillo is killed by Tony Camonte, setting off gang wars over the control of Chicago's bootlegging business.  Under orders from their boss Johnny Lovo, Tony and Guino Rinaldo terrorize South side bars to maintain it as Lovo's territory.  Afterward, they go on a several month long shooting spree, killing innocent bystanders as well as intended victims.  When Tony kills O'Hara, the North side boss, Lovo becomes scared.

Poppy, Lovo's mistress, visits Tony, and he shows her the neon Cook's Tours sign outside his window that has become his slogan: "The World Is Yours."  Tony takes over the North side, and goes on another shooting spree.  On St. Valentine's Day, seven gangsters are lined up in a garage and shot execution style.  After Tony kills the last of the big gang leaders, he goes to the Paradise Club, where he sees his sister Cesca dancing with a man.  In a jealous rage, Tony takes her home and beats her.

Then, when he leaves, he is chased by unknown gangsters.  Both cars go over the side of the road, but Tony survives.  When he finds out that Lovo set him up, Tony and Guino kill him, then Tony and Poppy hide out in Florida for a month.  While they are gone, Guino and Cesca fall in love and marry.  Tony returns to find Guino in Cesca's apartment and kills him before she can explain that they were married.

A short time later, the police surround Tony's apartment, and he and Cesca fight them off until she dies of a gunshot wound.  Finally, Tony surrenders after his room is inundated with tear gas and he cannot stand to be alone.  At the last minute, he makes a dash for freedom, but is gunned down by the police and dies under the Cook's Tours sign.