Set during the first stage of the Hundred Years' War, it follows Thomas of Hookton's quest to find the Holy Grail, a relic which will grant decisive victory to the possessor.

Unknown to them, the ambitious French Cardinal Bessières has dispatched Bernard de Taillebourg there also to find the Holy Grail.

Nearing Durham, Thomas is caught up in a Scottish raid taking advantage of the main English forces being away fighting in France.

A French priest brings word from Sir Guillaume asking for Thomas's help; he has been outlawed (because of his actions in the Battle of Crécy) and his castle is under siege.

Jeanette fled the Battle of Crécy, mistakenly thinking (as did everyone else) that the French would prevail, and thus no longer has the Prince of Wales' favour.

Charles has four fortified camps built, one before each of the city's gates, and gives his subordinates strict orders to remain inside their particular fortification until he gives the signal.

It works at first; Dagworth is drawn into a deadly ambush when he launches a night attack on the largest of the fortifications, the one under the command of Charles, and his men suffer heavy casualties.

However, just when all seems lost, the garrison sorties and takes the French by surprise, routing them and eventually winning the Battle of La Roche-Derrien.