The year is 1141 and civil war continues to rage. In the battle of Lincoln the sheriff of Shropshire is captured and the king himself taken prisoner by his enemies. An exchange of valuable prisoners is arranged and the sheriff is to be recovered in place of a young Welsh lordling, who in turn had been captured in the course of a misguided assault on a convent.

But before the exchange can be completed, one captive is murdered. To Brother Cadfael, who first notices the evidence of unnatural death, falls the task of gathering enough clues to prove it.