AD 255: Ballista, aged thirty-four, now an eques and a distinguished soldier in the service of the Emperor Valerian and his son Gallienus, is appointed Dux Ripae, the military commander of the Empire's eastern frontier, between the Euphrates and Tigris rivers.

His task is to prepare the small fortified town of Arete, on the banks of the Euphrates, for an attack by the invading armies of the Sassanid Persians, under Shapur I.

There, in addition to the shortage of troops, he is forced to cope with the arrogance of his subordinate officers (who, since he was originally a diplomatic hostage from the Angles tribe, consider him a barbarian), and the divides between the various religious, national and political factions that control the town's government.

On that night, a fanatic Christian priest, inviting punishment of all the "sinners" inside the town, guides the Persians to a tunnel underneath the walls, and the city is overrun.

As he watches Arete burn from afar, Ballista reflects that he has failed to hold the town, but bitterly realizes that he was never meant to; the Empire is already engaged in other wars on two different fronts, and no relief force was dispatched to the East.