It is the last in a sequence of novels, chronologically started with The Eagle of the Ninth, loosely tracing a family of the Roman Empire, then Britain, and finally Norse-Britain, who inherit an emerald seal ring bearing the insignia of a dolphin.

Bjorn is fostered to a Harp player, an old man who originally fought the Normans at Hastings, who refers to being part of that Shield Wall resisting William the Conqueror.

The instrument becomes a key feature of the narrative, being a symbolic link to the indigenous Welsh and British peoples with whom the Norse of Lakeland have intermingled.

They succeed in infiltrating the enemy camp, and gather much information, but are ultimately discovered when the emerald seal ring is recognized by a Norman knight who had previously had a life-and-death struggle with Bjorn.

[2] Although her own inimitable take on the story of Jarl Buthar's guerilla campaign and a final battle at Rannerdale between the Normans and the Anglo-Scandinavian Cumbrians led by the Jarl, Sutcliff's novel was clearly inspired by the dramatized history written by Lakeland historian Nicholas Size, called "The Secret Valley: The Real Romance of Unconquered Lakeland" (pub.