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.

The price of Caesar's mercy: Cicero nominates Caesar to an unprecedented ten-year dictatorship and Brutus speaks in support of the motion. With martial authority and Niobe at his side, Vorenus campaigns for magistrate under Posca's advice, but is shocked to learn from Posca that Caesar has already fixed the election in Vorenus's favor.
Meanwhile, Atia continues her humiliation of Servilia while denying any involvement in her assault.

Lucius Vorenus, the power-hungry 'Son of Hades', controls the Aventine but practices a pointless, beastly reign of terror, coming down hard on gang captains for futilities, and nearly ruins his friendship with Pullo, whose efforts to calm things down are mistaken for disrespect and disobedience, even starts a fight once he learns the truth about Niobe; Pullio leaves Rome disgusted.
After Atia convinces Marc Antony the governorship of ghastly Macedonia after his consulate would not only be unpleasant but leave him exposed to his dangerous enemies.

Vorenus and Pullo return to the Collegium in Rome with Vorenus' family, but some of the 'changes' that Pullo had warned him about regarding his two daughters Vorena both the Elder and Younger, as well as his wife's son, begin to manifest themselves into hostility and resentment.
But Vorenus is completely oblivious to this, and is too blissfully happy to return to the role of 'father'. Meanwhile, Octavian finally returns and manages to negotiate the Senate Consul's seat from a scheming Cicero.

Cicero is finally undone by his duplicity in the form of Titus Pullo, acting upon Octavian's request to assassinate all supporters of Brutus in Rome. Pullo and Vorenus enjoy a happy if counterfeit outing with their families in the countryside to hide their assassin mission.
Vorena the Elder's dangerous dalliance with one of Mimeo's men continues, and the torrid affair between Octavia and Agrippa escalates. Atia's casual cruelty to ask for the assassination of people she doesn't like results in complete disaster for Octavia's friend Jocasta.