Set in Britain during the 2nd century A.D., it's the story of a wounded Roman soldier and his Celtic slave who embark on a perilous journey into the far north to discover what happened to the Ninth Legion, which went on a mission into the wilds of Scotland 15 years earlier and never came back.

The soldier's father was the legion's general, and his disappearance, along with the loss of the legion's symbol, the eponymous eagle, cast a stain upon his family.