St. Mary's Priory, situated on a bleak and miserable moor, is being torn apart by rumours and rivalries. Lady Elizabeth of Topsham, Prioress, struggles to maintain both her position and her community.

Sister Margherita, her treasurer, has accused Elizabeth of lascivious disregard, claiming that she has given money needed for essential repairs to the new vicar - a man Lady Elizabeth often sees alone at night. With her support amongst the other nuns growing, and a sister murdered in the infirmary, suspicion falls upon the Priory.

Sir Baldwin de Furnshill, Keeper of the King's Peace, and his friend Bailiff Simon Puttock are summoned to investigate. With the nuns breaking their holy vows of obedience, chastity and poverty with alarming frequency, and the discovery of a second murder, Baldwin and Simon face their most difficult case yet.