On the Rhine frontier in Germania, a new draft of recruits arrive at the base of the Second Legion. One young recruit, Cato, bears a letter from Emperor Claudius appointing him to the rank of Centurion, however, since Cato is barely seventeen, the Legion's commander, Vespasian, proposes a compromise by appointing him optio, the centurion's second-in-command. The letter also contains another message, written in code, warning the legate that someone on his staff has been implicated in Scribonianus's failed coup attempt, and the Emperor has sent an unnamed Imperial agent to identify the person.

Cato, the son of a freed slave at the Imperial Palace, is unused to the harsh life of a Roman Legion, and is resented by his fellow recruits for being promoted ahead of them. He sticks through basic training through determination, earning Centurion Macro's grudging approval.

When a Roman tax collector is mutilated by the chief of a local German town, Vespasian sends the Third Cohort, under tribune Vitellius, which includes Macro's Sixth Century. Because Macro's regular optio is laid up by an injury, he conscripts Cato to replace him. No one is expecting any trouble, but once the Cohort is inside the village, they are ambushed by a much larger force of Germans hiding in the woods. The Cohort barricades itself behind the village's walls, with no option except to hold out until they are reported overdue at the base and Vespasian arrives with reinforcements.

At Cato's suggestion, the embattled legionaries set fire to the attacking Germans' rudimentary battering ram, but the flames unfortunately spread to the gate, then the rest of the village, forcing the century to fall back from the walls. Whilst in retreat to the main village square, Macro and Cato are cut off from their comrades and the former is hit by a javelin. Ignoring Macro's command to leave him behind, Cato charges alone at the attacking Germans, driving them away with the Century's Standard (Vexillum), long enough to help Macro to his feet and drag him into a nearby building. With a combination of guile, improvisation, and sheer bravery, Cato manages to get himself, Macro and the standard through the fire and back to the Roman lines. The Germans fall back when Vespasian arrives unexpectedly early, leading reinforcements. Macro later nominates Cato for a Grass Crown, and, to their shared embarrassment, both are invited to dine at Vespasian's house after the Cohort returns to base.

Whilst recovering from his wounds, Macro asks Cato to teach him to read and write; all centurions are required to process paperwork, but Macro has, until then, been concealing his illiteracy by delegating his work to a wizened clerk, Piso.

The Legion soon receives new orders, to march into Gaul to join the force mustering for Claudius's invasion of Britain.