The story of America's most notorious gangster, Al Capone, starts on the streets of Brooklyn. In 1913, at the age of 14, Capone starts running errands for local gangsters - just one of many immigrants struggling to survive. One day, young Al Capone gets caught in a police raid. He quickly grabs what money he can, evades police, and runs to the headquarters of mob boss Johnny Torrio. Torrio rewards Capone for his quick-thinking. When Torrio recognizes Capone's intelligence and drive, he takes him under his wing.

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.

After the murder of his brother Frank, Al Capone is out for revenge against William Dever and the Chicago police. He plots to kill every undercover cop that was in Cicero on the day of Frank’s death. Before he can set his plan in motion, however, Johnny Torrio returns from Italy and reclaims his control of their criminal empire. Torrio forbids Capone from killing cops, fearing it would spark an all-out war. Despite Capone’s personal loss, Torrio returns to a booming business that’s raking in $10 million a month. However, as Torrio and Capone’s empire grows, it becomes harder to keep smaller factions under control. After a small group of Italian mobsters attacks one of Dean O’Banion’s distilleries, O’Banion blames Torrio and decides to end their business partnership. He announces his retirement to Torrio and offers Torrio an opportunity to buy him out. Torrio meets O’Banion at the Seine Brewery on May 19, 1924, with the buyout money. Chicago police raid the brewery and arrest Torrio while letting O’Banion walk free: O’Banion had set the whole thing up. With prior convictions, Torrio serves a mandatory prison sentence but is first released on bail. He vows to kill O’Banion as payback and gives Capone his blessing to kill O’Banion. Capone spends months planning the hit. Capone knows that catching O’Banion off guard is nearly impossible, so he places an order with O’Banion’s flower shop for nearly $20,000 worth of flowers, knowing that O’Banion personally oversees his biggest arrangements. On November 10, 1924, O’Banion works in his shop preparing the flower delivery. Two of Capone’s men enter the shop and fatally gun O’Banion down. Four days later, Chicago’s most powerful Irish gangster is laid to rest in one of the most lavish funerals Chicago has ever seen. More than two dozen cars are required to transport floral arrangements as a crowd of 10,000 mourns the death of the infamous gangster. Torrio and Capone attend the funeral, infuriating O’Banion’s top lieutenants, Hymie Weiss and Bugs Moran. Chaos breaks out as smaller gangs through the city begin a turf war. It’s the beginning of an era of bloodshed in Chicago known as the Beer Wars. Over 300 people are murdered at a rate of 75 to 100 murders a year.

Tony Accardo have taken the reins of the criminal empire built by their mentor Al Capone. Thanks to Sam Giancana's takeover of the policy racket, the gangsters are now making millions. With so much money coming in, Accardo knows they need a place to hide it. They find their answer in a growing city where gambling is now legal: Las Vegas. However, the New York mafia is already there, led by criminal mastermind Meyer Lansky.

Even though his wife and children are avenged by the Erastes killing, Lucius Vorenus keeps mourning for over a month till Pullo gets Marc Antony who scolds him for Caesar's death and the continuing bloody Aventine gang riots that have resulted in the gangs wanting to fill the void left by Erastes sudden demise.
Octavian is loosing patience with Mark Antony for payment of Caesar's inheritance. Meanwhile, Queen Cleopatra of Egypt arrives in Rome to pay her respects to Caesar and she also negotiates with Antony's military protection for her Pharaonic throne.

Rejected by Vorenus and Eirene and racked by guilt for the killing of Eirene's fiancé out of a jealous rage, Pullo has sunk to working as an assassin for the gangster Erastes. When Pullo gets arrested for killing one of Caesar's popular opponents, he condemns himself by refusing to name who hired him, despite Octavian's attempts to defend him.
Meanwhile, Vorenus finds his new official duties as magistrate tedious, especially when he gets caught between Caesar and the demands made by veterans' spokesman Mascias.