Discharged because of a battle wound that has left him lame in one leg, young Roman officer Marcus Flavius Aquila tries to discover the truth about the disappearance of his father's legion in northern Britain.

Disguised as a Greek oculist and travelling beyond Hadrian's Wall with his freed ex-slave, the British native hunter Esca, Marcus finds that a demoralized and mutinous Ninth Legion was annihilated by a great rising of the northern tribes.

Sutcliff wrote in a foreword that she created the story from two elements: the disappearance of the Legio IX Hispana (Ninth Legion) from the historical record following an expedition north to deal with Caledonian tribes in 117; and the discovery of a wingless Roman eagle in excavations at Calleva Atrebatum (Silchester).

At the time Sutcliff wrote, it was a plausible theory that the unit had been wiped out in Britain during a period of unrest early in the reign of the Emperor Hadrian (AD 117–138).

[2] Scholarly opinion now disputes this, for there are extant records that have been interpreted as indicating that detachments of the Ninth Legion were serving on the Rhine frontier later than the year 117, and it has been suggested that it was probably annihilated in the east of the Roman Empire.