In Gaul in 52 B.C., two Roman soldiers, Legionary Titus Pullo and Centurion Lucius Vorenus, are tasked with recovering Julius Caesar's personal Eagle, stolen from his camp in the dead of night. With his campaign in Gaul coming to a successful conclusion, Caesar's popularity is continuing to grow. He's saddened however when he receives news from his good friend Pompey Magnus that his daughter, Pompey's wife, has died in childbirth. In the Senate, Pompey must defend the prolonged absence of his friend and co-Consul Caesar against charges of corruption and of waging an illegal war. It's all a ruse however as he is plotting to eliminate him. Meanwhile, Atia of the Julii sends her son Octavian to Gaul deliver a gift of a beautiful stallion to his great uncle Julius Caesar. He is taken prisoner along the way. Fortunately, Vorenus and Pullo rescue him and as a result, both receive Caesar's favor.

Caesar winters his army in Gaul near the Italian border while his enemies in Rome fear that he will do something no Roman General has done: cross the Rubicon with his army. To counter-balance his opponents, Caesar puts his friend Mark Antony up for office as the people's Consul. Vorenus and Pullo accompany Mark Antony and Octavian to Rome. For Pullo, it's an opportunity to get drunk and ravish every woman in sight. He also loses all of his money in a crooked dice game leading to a major brawl. For Vorenus, he will have a reunion with his wife Niobe who he has not seen for 8 years. He's told that the baby he finds in his home is his grandson but Niobe is hiding a secret. In the Senate, there is a move to have Caesar declared a traitor but when Mark Antony tries to return to the Senate to veto their decision, Pullo is attacked by someone he fought the night before. Antony thinks the attack is against him and Pullo kills the man and Caesar declares Pullo a hero.

With Caesar quickly advancing toward Rome, Pompey begins ordering a retreat and forces his citizens to pick sides. Timon does Atia's dirty work and Vorenus cannot understand why no one is resisting while on a scouting mission.

Atia and her family have stayed behind when Pompey and others fled and she now throws a party for her triumphant uncle, Julius Caesar. She clearly has her sights set on the great leader and is puzzled when he sends her a guest list that includes Servilia of the Junii. She soon finds out why. Pompey wants to find out what happened to the gold from the Roman Treasury and sends his son Quintus to Rome to find out what happened. They go directly to Vorenus' home but he has no idea what they are talking about. Pullo arrives however and Vorenus tells him to go directly to Caesar himself and explain. Octavian meanwhile learns of one of his uncle's secrets.

Caesar contemplates Pompey's counteroffer. Vorenus continues to struggle as a businessman and must reevaluate his career choice. Atia schemes to come between Caesar and Servilia and Pullo seeks Octavian's help.

As they try to reach Caesar in Greece, the shipwrecked Vorenus and Pullo confront an unexpected dilemma. Back in Rome, Atia again sends Octavia to beg protection from Servilia, who complies readily to encourage her growing friendship with Octavia. As they prepare their counterattack against Caesar, Pompey's Republicans begin to squabble over both the anticipated spoils and the method of finishing Caesar off. Niobe's sister Lyde comes back to see her and comforts her over Vorenus's disappearance. Badly outnumbered, Caesar and Antony make their stand at Pharsalus. On his arrival in Egypt, Pompey is greeted by a Roman veteran.

The defeated Cato and Scipio fall back on the African city of Utica after the Battle of Thapsus where they decide to end it all. Caesar, after two years of fighting in Greece and Egypt, returns to Rome and receives a hero's welcome. Meanwhile, Servilia's cold rage against the Julii clan makes Brutus's submission to Caesar even more painful. Octavian returns home from military school and Caesar, impressed with his nephew's incisive political acumen, appoints him a pontiff despite his youth. Having opened and operated a profitable butcher shop, Niobe and Lyde get the reluctant Vorenus and Pullo to join the trade, but Vorenus again crosses paths with the thug Erastes, now the crime lord of the Aventine. Learning of Caesar's secret "affliction," Servilia persuades her lover Octavia to cajole her brother into revealing more, but she only learns Niobe's secret instead. Servilia motivates Octavia to try again by other means, with dreadful consequences for both sister and brother. Pullo's affection for his slave Eirene deepens. Enraged at Servilia's manipulation of her children, Atia enlists Timon and his band of thugs to retaliate, thus escalating the term 'catfight' to a very different level.