The story of the Arthurian legend, based on the 'Sarmatian hypothesis' which contends that the legend has a historical nucleus in the Sarmatian heavy cavalry troops stationed in Britain, and that the Roman-British military commander, Lucius Artorius Castus is the historical person behind the legend.

Britain, A.D. 117. Quintus Dias, the sole survivor of a Pictish raid on a Roman frontier fort, marches north with General Virilus' legendary Ninth Legion, under orders to wipe the Picts from the face of the Earth and destroy their leader, Gorlacon.

In 140 AD, twenty years after the unexplained disappearance of the entire Ninth Legion in the mountains of Scotland, young centurion Marcus Aquila arrives from Rome to solve the mystery and restore the reputation of his father, the commander of the Ninth. Accompanied only by his British slave Esca, Marcus sets out across Hadrian's Wall into the uncharted highlands of Caledonia - to confront its savage tribes, make peace with his father's memory, and retrieve the lost legion's golden emblem, the Eagle of the Ninth.