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.

Then Ruso's recently arrived clerk, Candidus, goes missing. A native boy thinks he sees a body being hidden inside the wall's half-finished stonework, and a worrying rumor begins to spread. When the soldiers ransack the nearby farms looking for Candidus, Tilla's tentative friendship with a local family turns to anger and disappointment. It's clear that the sacred rites to bless her marriage to Ruso will have to wait. Tensions only increase when Branan, the family's youngest son, also vanishes. He was last seen in the company of a lone and unidentified soldier who claimed he was taking the boy to see Tilla.

As Ruso and Tilla try to solve the mystery of the two disappearances—while at the same time struggling to keep the peace between the Britons and the Romans—an intricate scheme involving slavery, changed identities, and fur trappers emerges, and it becomes imperative that Ruso find Branan before it's too late.