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.

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.

Caesar pursued Pompey to Greece but the tide has turned and it's Caesar who is on the run. He contacts Mark Antony, who he left in charge in Rome, to come and join him with the 13th Legion but Antony seems to be in no hurry. Vorenus has re-enlisted in Antony's Legion and he and Niobe have begun to renew their marital relationship. The presence of Niobe's sister is the only sour note in their house. Pullo meanwhile continues to train young Octavian in the use of arms and the boy's mother Atia is so pleased with his development that she decides he should take another step to manhood and orders Pullo to take him to a brothel so he can have his first sexual experience. Atia also continues with her scheming ways and believing that Caesar is done for, decides she had better make friends with Servilia.