AD305: Under the rule of the tetrarchic emperors, the Roman world has been reborn after half a century of chaos. But far away at the northern frontier of Britain, newly-promoted centurion Aurelius Castus feels very distant from the glories of empire. When the king of the Picts, the savage people beyond Hadrian’s Wall, dies in mysterious circumstances, Castus is selected to command the bodyguard of a Roman envoy sent to negotiate with the barbarians. Betrayed and surrounded in enemy country, he faces not only the violence of the Picts, but also a sinister imperial spymaster and a formidable chieftainess with a lust for power.

Desperate for revenge, Castus soon finds that the tendrils of imperial intrigue stretch even to the furthest frontiers. The coming campaign against the barbarians will test his loyalty as well as his courage, and see the rise of a new power: Constantine, a ruler destined both to reshape the Roman empire and to change the history of the world.