Thomas of Hookton is a seasoned veteran of King Edward's army and a natural leader in what will be remembered as the Hundred Years' War. Accompanied by a small but able band of soldiers, Thomas is sent to Gascony to capture the castle of Astarac. But he has ulterior motives for accepting the charge: Gascony is the home of the black knight, Guy de Vexille, who brutally slaughtered his father. It is also reputed to be the place where the Grail was last seen.
While capturing Astarac, Thomas learns of a tragedy in the making: a beautiful young woman named Genevieve, innocent if not pious, is to be burned as a heretic. Thomas prevents the corrupt local priest from carrying out his "God-given" duty, a sacrilege that turns him into an outcast, even among his own men. Eventually he and Genevieve have no choice but to flee. While hidden away at a monastery, they learn of a plot involving the creation of an imitation Grail for a diabolical end, and they witness the murder of a trusted priest at the hands of Guy de Vexille.
At last reconciled with his allies, Thomas leads his brave band in a bloody battle to the death—the outcome of which could determine the seat of power and the direction of Christendom forevermore.