In 119 A.D., the Ninth Roman Legion marches north into the wilds of Britain--and disappears without a trace. Fifteen years later, Marcus Flavius Aquila, the son of the unit's commander, embarks upon a quest to recover the lost eagle standard of the Ninth.

Threatened by an unstoppable tide of invaders, the last of the Roman Auxiliaries are setting sail to leave Britain forever. But Aquila, a young legionnaire, chooses to stay behind, in order to join the fight to save his native land.

When they accidentally uncover a plot against the British emperor Carausius, a young army surgeon and his soldier cousin are caught up in a maze of intrigue that leads them into an underground organization of secret agents in the service of Rome.