In less than two years, Al Capone has risen from a humble bookkeeper to second-in-command of Johnny Torrio's criminal empire in Chicago. The empire is worth the modern-day equivalent of $35 million. Capone purchases a home for his family and moves them from New York to Chicago.
Torrio and Capone expand their empire from Chicago's South Side to the North Side, which is controlled by Chicago's second largest gang, the Irish. Violence erupts as ruthless Irish gang leader Dean O'Banion protects his territory and bootlegging business from Torrio and Capone.
After several weeks of violence, Torrio meets with O'Banion and proposes a plan: O'Banion sells whiskey in the South Side, Torrio sells beer in the North Side, and they share a cut of each other's profits. Despite his reservations, O'Banion accepts Torrio's offer, creating the largest bootlegging syndicate in the city.
Having cemented his alliance with the North Side, Torrio moves to the next phase of his plan: buying off the city's politicians and police, including Mayor Big Bill Thompson. Gangs such as Torrio's and O'Banion's dedicate nearly 10 percent of their revenue to payoffs.
With politicians and cops in their pocket, the Italian and Irish conglomerate begins killing off Chicago's small gangs. As the death toll rises, Chicago's citizens decide they've had enough and elect William Dever as mayor. Dever vows to clean up Chicago by ridding the city of violence and corruption.
Knowing that Dever will not accept bribes, Torrio and Capone send a $100,000 cash bribe (equivalent to $1 million today) to Dever's chief of police Morgan Collins. Provoked by Torrio and Capone's attempt to corrupt his administration, Dever shuts down their breweries and distilleries and raids the Four Deuces, their headquarters and highest-grossing brothel.
In 1923, Torrio moves his operations to the quiet, suburban town of Cicero, Illinois, where Dever and Chicago police have no jurisdiction. Torrio puts Capone in charge of setting up new businesses to replace their shuttered Chicago businesses, including a new headquarters and casino at Cicero's Hawthorne Inn. Only weeks later, Torrio goes to Italy to care for his sick mother. Capone, at the mere age of 24, is left in charge of rebuilding Torrio's empire.
Capone enlists his brothers Frank and Ralph to help run the new businesses. He puts Ralph in control of day-to-day operations such as bookkeeping, hiring and firing. He charges Frank with buying off Cicero politicians. Now under the control of Capone and his brothers, the once sleepy town of Cicero begins to transform into a destination for drinking and entertainment.
As Capone's reputation grows, O'Banion demands a cut of the profit made from the new businesses in Cicero. Capone refuses but later changes his mind, in order to preserve the partnership with the Irish that Torrio had set up.
As Cicero transforms into a mob paradise, citizens begin objecting to the corruption of their town. Capone realizes that the politicians on his payroll could be voted out at the next election and tells Frank to do whatever it takes to keep their friends in office.
On election day, Frank oversees an effort to coerce citizens into voting for Capone's politicians. He also stuffs ballot boxes, kidnaps election officials and shoots up election headquarters.
As chaos breaks out in Cicero, town officials reach out to Dever. With Cicero out of his jurisdiction, Dever instructs an army of Chicago police officers that for the next 24 hours they no longer serve the city of Chicago. He dispatches them to Cicero as civilians who've been deputized to uphold the law in Cicero.
In Cicero, the plainclothes Chicago police officers shoot Frank dead in broad daylight. Some witnesses say Frank drew his gun first, while others believe the cops targeted him from the beginning. Capone's politicians win the Cicero elections but at a heavy cost to the Capone family. With his family in mourning, Capone plots his revenge, determined to war with Chicago police and Dever.