In the early twelfth century, civil war rages between the English and the Welsh. In the battle of Lincoln, the Welsh capture the sheriff of Shropshire and barter him for a young Welsh lord, who had been captured during a misguided assault on a convent. But before the exchange can be completed, one of the prisoners mysteriously dies.

Brother Cadfael suspects foul play—his theory hinging on a single thread that binds together the destinies of an unusual trio of young lovers. The unraveling of their tale yields a powerful subtext that explores the nature of love, identity, and divided loyalty.