Anno Domini 1369. The much loved Queen Phillippa lies dying at Windsor, and in the city of York the harvest has failed and the plague has returned. Welcome to the world of Owen Archer. In the atmosphere of fear and superstition that grips the city, rumors spread that the spate of deaths at St. Leonard's Hospital is no accident. Several "corrodians" - elderly people who have paid a dear sum to St. Leonard's hospital to care for them until they die - have died suspiciously, and there has been a number of thefts from the hospital as well. Sir Richard de Ravenser, Master of the Hospital, returns from Westminster painfully aware that scandal could ruin the hospital and his own career. Anxious to address the crisis, he presses one-eyed spy Owen Archer into service to investigate the mysterious goings-on. With plague rife and the city's inhabitants daily besieging his apothecary wife for new preventatives and cures, Owen is unwilling to become involved. There is too little to link the victims to one another; the riddle seems unsolvable. But careful inquiries reveal a further riddle, spoken by one of the dead, that might be the key Owen needs. Three seemingly separate mysteries intertwine and Owen Archer must solve them all if he is to decipher the Riddle of St. Leonard's.