Obsessed with discovering the fate of his father's lost legion, Marcus arrives in Britain to take command of a frontier fort. He makes friends with Cradoc, a British warrior, and all seems peaceful. Then, on the night of the full moon, the tribes rise in rebellion to attack the fort.

Badly wounded in the battle, Marcus is discharged from the legion, his soldiering at an end. He goes to recuperate with his uncle Aquila in Calleva. There he saves Esca, a British gladiator, from death in the arena, and from him he hears of the lost legion.

His wound healing after a painful operation, Marcus considers where his future now lies. A Roman commander visiting his uncle tells him of a rumour that the Caledonian tribes are about to rise and that the Eagle of the Ninth is in their hands. Marcus and Esca go north to recover it.

After months of searching, Marcus and Esca meet Guern, the hunter. Now living as a British tribesman, Guern confesses that he was once a centurion with the Ninth, and tells them how the legion was annihilated. Later, at a tribal ceremony, Marcus sees the Eagle.

Marcus learns of the legion's last stand and meets the man who killed his father. He conspires with Esca to steal the Eagle but fleeing with it, they discover the tribesmen are close behind them.

With the help of the centurion turned British tribesman Guern, Marcus and Esca manage to evade the Epidii and escape across Hadrian's Wall with the Eagle. Having returned to Roman territory, Marcus seeks to reassemble the Ninth Legion.