It's 1321 and Lady Elizabeth of Topsham, prioress of St Mary's, is fighting to retain her position in the face of an accusation from Sister Margherita, the treasurer, that she has given much-needed funds to the new vicar, a man she often sees alone - at night. Many of the nuns are convinced that Margherita would make a better prioress - especially now a young nun has been found murdered on her sickbed. Sir Baldwin Furnshill, Keeper of the King's Peace and his old friend Bailiff Simon Puttock are summoned to investigate and when a second nun is murdered, they find themselves facing their most difficult case yet.