Anno Domini 1369. The much loved Queen Philippa lies dying at Windsor, and the plague has returned to the city of York. In an atmosphere of fear and superstition, rumours spread that a spate of deaths at St Leonard's Hospital in York is no accident. The hospital is in debt and has suffered thefts: Sir Richard de Ravenser, Master of the Hospital, returns from Winchester painfully aware that scandal could ruin his own career. Anxious to avert a crisis, he requests the services of Owen Archer, spy for the Archbishop.

With plague rife and the city's inhabitants besieging his wife, the Apothecary, for new cures, Owen Archer is unwilling to become involved. There is too little to link the victims to each other: the riddle seems unsolvable. But careful enquiries reveal a further riddle, connected to one of the victims. Is this where the truth lies?