At Midsomer Priory,home to a small group of nuns,Mother Thomas Aquinas is strangled and,shortly afterwards,a set of silver worth £60,000 is found to be stolen from the safe. Sister Catherine,the most recent member of the order,turns out to be descended from Sir Anthony Vertue,who donated the priory to the nuns two centuries earlier and Barnaby learns that if the order is ended she will inherit the land and buildings. Father Behan,who heard the nuns' confessions and had long exhorted Mother Julian,the prioress,to sell the premises,is also killed and the missing silver turns up,not stolen but sold by Mother Thomas Aquinas. Then Barnaby learns of an atrocity in Africa thirty years earlier where Mother Julian was a missionary,which could help him solve the case but also put an end to the sacred trust.