The Roman Empire is on the brink of civil war. Only Maxentius, tyrant of Rome, stands between the emperor Constantine and supreme power in the west. Aurelius Castus, promoted from the ranks for valour, is now a tribune in Constantine's army. But as Constantine becomes increasingly devoted to Christianity, Castus is forced to ask himself whether he is backing the wrong man... The coming war will decide the fate of empire but also expose shadowy realms of treachery at the heart of Rome itself.