Al Capone moves back to Chicago and takes over two floors of the city's finest hotel, the Lexington. With a thriving business and Chicago politicians in his back pocket, Capone soon reaches a level of celebrity normally reserved for athletes and entertainers. As Capone's profile grows, he begins to indulge in all that the Roaring '20s have to offer, including cocaine. While Capone enjoys his reign at the top of the Chicago underworld, Irish gangster Bugs Moran seeks to take him down as revenge for the deaths of his partners Dean O'Banion and Hymie Weiss. On March 7, 1928, Moran orders a hit on one of Capone's top associates, Jack McGurn, the man who took out Hymie Weiss. McGurn survives but Capone decides to retaliate by taking out Moran and as many of his men as possible - all at once. On February 14, 1929, Capone's men dress as law enforcement and raid the liquor buy. They order Moran's men to face the wall for handcuffing, but while their backs are turned, Capone's men shoot all of them. Firing 70 rounds of ammunition in less than 10 seconds, the assassins carry out the biggest gangland hit in American history that will come to be known as the St. Valentine's Day Massacre. Through a stroke of luck, Moran avoids the massacre, but knows he's been defeated.

In 1927, Prohibition agent Eliot Ness narrowly escapes a hit ordered by Al Capone. Determined to prove to Capone that he's not intimidated, Ness and his team of Untouchables execute a series of high-profile raids on Capone's breweries. Over the course of a few months, they destroy more than 200,000 gallons of beer, worth a modern-day equivalent of $134 million - and the newspapers cover it all. Capone lashes out against Ness in the press. What Capone doesn't realize is that Ness and the Prohibition agents aren't the only ones trying to take him down. For the past four years, another government agency with more manpower and resources than Ness has been going after the infamous kingpin: the Internal Revenue Service.