Instead of leaving with the last of the Roman legions, Aquila, a young officer, decides that his loyalties lie with Britain, and he eventually joins the forces of the Roman-British leader Ambrosius to fight against the Saxon hordes.
A young Roman army medical officer, sent to Britain during the period of waning Roman rule, befriends a kinsman with whom he shares an adventure of intrigue, exile, and underground activity with the Lost Ninth Legion.