The Chicago mob of the 1920s is an ironic organization; one one hand, they value loyalty above everything else. On the other, its members are constantly plotting against each other and forging alliances that they hope will enable them to unseat the Capo and install them as his replacement instead. Such is the case when Italian mafiosi John "Johnny" Lovo instructs Italian immigrant Antonio "Tony" Cammonte to assassinate "Big" Louis Castillo, Boss of the city's South Side. Johnny then takes control of the South Side and makes Tony his number one lieutenant. They exploit the city's state of prohibition and make a fortune selling illegal beer to speakeasies, and undercutting rival syndicates who are already selling to other bars. However, as much as he wants to be the top Italian mafiosi, Johnny warns Tony that he should never get entangled with the Irish mobsters. They are led by a gangster called O'Hara. He runs the North Side in much the same way that Johnny runs the South, but Tony takes no notice of his boss's instructions, deliberately shooting up bars that O'Hara owns and in the process coming to the attention of both rival gangs and Chicago's law enforcement. Johnny realizes that he now has no control over Tony and that his lieutenant is actually planning to get rid of him in the same way he dispensed with Castillo, and take over his position.
Tony is not only interested in taking Johnny's position at the top of the syndicate. He has also set his sights on Johnny's girlfriend Poppy. As his reputation increases, so does her interest in him. One evening he invites her to his apartment and he shows her what has been inspiring him to make his move both with her and within the organization; across from his apartment is a light-up billboard advertising a travel company that has the slogan 'The World Is Yours". This is what has been fueling his ambition. Eventually, Tony declares war on the North Side with the intention of taking it over for his own. He send his close friend Guino Rinaldo to a florist shop to kill O'Hara. The North Side retaliates. Armed with Thompson sub-machine guns, they fight back against Tony and the south. Their weapons captivate Tony and he becomes obsessed with them. He leads his own men to destroy the North Side and take over all of their business interests. He even impersonates a police officer and guns down his rivals dressed that way. He also kills the North's leader, Gaffney, as his is in the middle of making a strike at a bowling alley. Johnny is now absolutely certain that his one-time protege is making a bid to take over. He arranges Tony's assassination but Tony miraculously manages to escape this attempt on his life, and he and Guino succeed in killing Johnny, which makes Tony the undisputed boss of the city, running both the South and the North sides.
Previously the different syndicates had operated under the public radar, but Tony's escalation in violence has alerted the public to the way in which organized crime seems to run the city. There is a demand for the police to put a stop to the violence and the crime. The police begin to close in on Tony. His mental state is not improved when he sees his beloved little sister Francesca with Guino. He kills his best friend in a fit of jealous rage, before Francesca has had time to tell him that they have actually got married in secret. She runs to the police and informs on her brother. They go to arrest Tony for Guino's murder, and Tony barricades himself into his apartment prepared for a shoot out. Francesca returns too, initially ready to kill him herself for murdering her husband, but she ends up helping him evade the police. In doing so she is hit by a stray bullet, and killed. The police fill the apartment with tear gas, and Tony escapes down the back stairs, but the police are waiting for him. Tony begs for his life but tries to make a break for it. The police gun him down, underneath the light of the billboard that flashes up the inspiration "The World Is Yours".
