Brother Cadfael sets out to visit the Saint Giles leper colony outside Shrewsbury, knowing that a grand wedding is due to take place at the Abbey of Saint Peter and Saint Paul. As he arrives at Saint Giles the nuptial party passes the colony's gates. He sees the fragile bride, looking like a prisoner between her two stern guardians, and the groom, an arrogant, fleshy aristocrat old enough to be her grandfather.

With his usual astuteness he suspects that this union may be more damned than blessed. He is horrifically proved right when a savage murder disrupts the May-December marriage leaving Brother Cadfael with a dark and terrible mystery to solve. For the key to the killing - and a secret - are hidden among the lepers of Saint Giles and Brother Cadfael's skills must diagnose a sickness, not of the body, but of a twisted soul.