Centurion Aurelius Castus has been promoted to tribune and seems settled in a life of comfort with his new wife and young son. But Rome is in the tyrranical grip of the usurper Maxentius, and Constantine is forming a plan for a surprise attack from the west, in order to take the city. Castus, who once saved the emperor's life, is called upon to play a key role in the campaign. Once again he is caught up in events far beyond his control, where nothing is quite what it seems, not even his new marriage.