In the battle of Lincoln, the sheriff of Shropshire is captured, and the king himself is detained by his enemies. An exchange of prisoners is arranged: the sheriff will be bartered for a young Welsh lordling who has been captured during a misguided assault on a convent. All seems to be going according to plan until one of the prisoners dies under mysterious circumstances before the exchange is completed. Cadfael must determine whether the death was accidental or contrived.