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

After ambushing a band of men-at-arms, Thomas and his men capture their leader, Joscelyn, the count's nephew and heir.

Sir Guillaume reluctantly allows Joscelyn to leave to persuade his miserly uncle to pay his ransom.

Realising his excommunication has lost him the support of many of his men, Thomas departs with Genevieve, leaving his friend Sir Guillaume in charge.

Planchard guesses why Thomas is in the region and shows him a small, old wood box made for the Vexilles; it is said to have held the Holy Grail, but it is now empty.

With Guy Vexille and Charles Bessières searching for him, Thomas has no choice but to try to return to Castillon d'Arbazon.

Thomas's men welcome him back, but they are still hopelessly outnumbered, and Joscelyn has brought a cannon to break down the gate and walls.

Joscelyn flees the sickness, but Guy Vexille gathers all the healthy soldiers who have stayed and launches an assault.

Back in the remains of Thomas's village of Hookton, he finds a crude clay bowl his father had used to hold the communion wafers.

Thomas himself had come upon the bowl after the sack of Hookton, and thrown it away as worthless, but it fits inside the Vexille box perfectly.