Medieval London comes vividly to life in this fourth investigation by Hugh Corbett, chief clerk to England's Edward I. In January 1299, at High Mass with the king and other nobles, Walter de Montfort, the Dean of St. Paul's Cathedral, falls dead, poisoned. Edward, whose recent ill-received attempts to tax the church to raise funds for his wars against France and rebel Scots make him a suspect, orders Corbett to investigate. With the aid, and often hindrance, of his dissolute servant Ranulf, Corbett discovers that the victim owned a house of prostitution and served as the fence for a gang of outlaws. The clerk comes to suspect, however, that despite the Dean's criminal activities, he was not the intended victim. Finding the key to the mystery in the ritual of the mass, Corbett gets his answer after another murder, although he himself almost perishes. Like previous Corbett mysteries, this one is based on a true incident.