52 B.C.E. The Roman legions under the command of a famed general Gaius Julius Caesar are fighting in Gaul, ready to subjugate the last vestiges of resistance from the local tribesmen and put an end to the eight-year long campaign. Among Caesar's forces is the famous XIII Legion, with its first centurion being Lucius Vorenus, a man known for his strict demeanour and rigid discipline. During the final battle, he masterfully leads his men into combat, maintaining the battle formation by whistling commands. The Gauls are unable to breach the wall of shields until one legionnaire, Titus Pullo, breaks formation and rushes straight at the enemy. Vorenus attempts to pull him back in only to get a punch to the face, but other soldiers knock Pullo down and take him to the back.
The battle is won, and at the camp Pullo receives a whipping for insubordination along with a death sentence the next day. The Gallic king Vercingetorix lays down his sword in defeat before Caesar and is then stripped of his clothes and forced to kiss the golden eagle standard, with the Roman soldiers shouting the leader's name in triumph. But the feeling of victory is short lived, as the general receives a letter from his old friend and fellow consul, Gnaeus Pompey Magnus, that relays that Julia, Caesar's daughter and Pompey's wife, has died due to the complications of childbirth, with her daughter being stillborn. His closest confidant, Mark Antony, comforts him, but another concern arises as Caesar realizes Pompey needs a new wife.
In Rome, plebeians are celebrating as spoils from Gaul are being distributed among them, while the members of Senate such as Porcius Cato the Younger, Marcus Tullius Cicero and Quintus Metellus Scipio are unhappy with Caesar's influence rising among the poor folk. Caesar's niece Atia purchases a white stallion for his uncle as a gift and sends her son Gaius Octavian to the camp to deliver it. At the Senate, Pompey defends his friends from the senators' accusations of treason and vetoes them. Scipio offers his recently widowed daughter, Cornelia, as a bride for the consul, while Cato tries to instill the thoughts of Caesar's betrayal into his mind and to make him renounce his old friend, but Pompey is unmoved.
During the night, several men coated in blue paint sneak into Caesar's camp and steal the eagle standard. The next morning, Octavian sets off for Gaul with a small number of warriors and slaves. Pompey is incensed that Atia purchased the stallion before him and sends his slave to handle the matter. Pullo is awaiting his execution, while Marcus Junius Brutus, Caesar's protégé and the son of his lover Servilla arrives to the camp, bringing news from Rome. Caesar tells Brutus about the standard being stolen, citing it as a reason of his troops' morale lowering.
Octavian's entourage is attacked by unknown assailants who kill everyone but him while he's taken prisoner. Antony comes to Vorenus and tasks him with finding the standard. The centurion begins by having some Gauls crucified to make them talk until one of them tells him about "Blue Spaniards" who took the eagle. He then goes to retrieve Pullo from prison and orders him to help in his search.
Brutus returns to Rome, giving Caesar's letters to Servilla and Atia, while Pompey asks him about the situation in the camp. Atia is tasked with finding a new wife for Pompey, with her choice falling on her daughter, Octavia, who has recently married a patrician named Glaubius. She tells the young woman to divorce her husband, much to Octavia's protests, and Pompey takes her despite the marriage being a month away.
During the night rest, Pullo and Vorenus's horses are stolen by bandits, but they happen upon Octavian being forced to drag a cart. The legionaries dispatch his captors with ease and free the boy who beats one of the slavers to death with a club. Octavian tells his saviors that the stealing of the standard is actually beneficial to Caesar, as he look weakened enough for Pompey to attack him and come off as an aggressor. They find traces of blue paint in the cart and realize that they happened upon "Blue Spaniards". A man escapes from the cart holding the standard, but Vorenus kills him with a thrown javelin. The eagle is retrieved and Octavian is brought to camp along with the stallion. Pompey's brand is discovered on the slain man's head, signifying the beginning of the conflict, as his head is brought back to Rome along with a message from Caesar: he is coming back. Spurned by this, Pompey decides to marry Cornelia while Octavia is berated by her mother, pining for Glabius to take her back and beginning to plot Pompey's death. The legions head to Rome while their camp burns behind, as the lines have been crossed.
- Blood Bath: Atia is showered with the blood of a sacrificial bull as part of a ritual to grant her son divine protection.
- Establishing Character Moment: We get quite a few for the main characters:
- Vorenus ls first seen whistling commands to his century and maintaining order of command, while Pullo breaks out of formation to show off his fighting skills, showing themselves to be a stickler for the rules and order and a troublemaker, respectively.
- Mark Antony is introduced watching Pullo's lashing while gleefully biting into an apple, a few minutes later he's comforting Caesar, establishing himself to be both a complete hedonist who is utterly loyal to Caesar.
- Atia unabashedly assigning a politically oriented errand to a man the moment the two finish having sex. Then she parades herself naked in front of her son after her bath.
- Green-Eyed Monster: Pompey is shown to have a degree of jealousy towards Caesar, clearly not satisfied with his lot in comparison to his friend's.Pompey Magnus: Damn him! Must he have everything?!
- Sex for Services: Instead of paying for the white stallion with gold, Atia has sex with Timon, and he's clearly not against the arrangement.
- Shameful Strip: Vercingetorix has his clothes stripped off by Roman soldiers, left bare in front of Caesar to be humbled and humiliated.
- Shameless Fanservice Girl: Atia has no reservations walking around her house naked, even in front of her own son.
- Snipe Hunt: Vorenus treats the search for the eagle to be an impossible task, taking Pullo with him as he's got nothing to lose.Lucius Vorenus: What do you think our chances are of finding the eagle?Titus Pullo: Thirteenth never fails, eh?Lucius Vorenus: Little or no chance. The eagle might be anywhere: buried, melted down, sunk to the bottom of the sea. We're hunting a black dog in the night. Not finding is much the same as losing it. We'll be disgraced. Now, I chose you because you are disgraced already and condemned to die.Titus Pullo: So you weren't making amends at all, then.Lucius Vorenus: Pullo, in effect, you're dead already. The loss of one drunken fool does little damage to the Legion.
- Villainous Gold Tooth: The head slaver who captures Octavian has several golden teeth which Pullo plucks out after killing him.
- Vitriolic Best Buds: Vorenus and Pullo quickly grow into this dynamic as they spend more time around each other, trading barbs while still looking out for one another.
