Season one begins in 52 B.C. as Gaius Julius Caesar completes his quest of Gaul after eight years of war and prepares to return with his army to Rome. But while Caesar's self-interested niece Atia and long-lost paramour Servilia anxiously await his return, the ruling patricians despair that Caesar's homecoming will disrupt the status quo. As Caesar's legions move closer to Rome, allegiances are put to the test for both soldiers and civilians — and the escalating tensions climax with a full-scale conflict destined to change history.

Season two's storyline picks up right where season one left off: with the 44 B.C. assassination of Gaius Julius Caesar and the near-simultaneous death of Niobe, wife of Caesar's faithful bodyguard Lucius Vorenus. Diverted from his post after learning that Lucius, his assumed grandson, was in fact Niobe's illegitimate son, Vorenus now descends into near-madness over his failure to protect Caesar, his role in his spouse's death, and the rumored murder of his estranged daughters.