The medicus Ruso and his wife Tilla are back in the borderlands of Britannia, this time helping to tend the builders of Hadrian's Great Wall. Having been forced to move off their land, the Britons are distinctly on edge and are still smarting from the failure of a recent rebellion that claimed many lives. When Ruso's recently arrived clerk, Candidus, goes missing, tensions increase. Then a local boy also vanishes, and Ruso and Tilla discover an intricate scheme involving slavery, changed identities, and fur trappers.