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 ...