It is 1321, and Lady Elizabeth of Topsham, prioress of St. Mary's, is fighting to retain her position in the face of devastating opposition. She has been accused by Sister Margherita, St. Mary's treasurer, of giving 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 that it is certain that Moll, a young nun, was murdered in her sick bed. Sir Baldwin Furnshill, Keeper of the King's Peace, together with his old friend Simon Puttock, are summoned to investigate. There is no doubt that the threefold vows of obedience, chastity, and poverty are being broken with alarming frequency. Then, when a second nun is murdered, they find themselves facing their most difficult case yet.