Brother Cadfael is confounded when his own potion is used to kill his former love’s new husband Gervase Bonel is a guest of Shrewsbury Abbey of Saint Peter and Saint Paul when he suddenly takes ill. Luckily, the abbey boasts the services of the clever and kindly Brother Cadfael, a skilled herbalist. Cadfael hurries to the man’s bedside, only to be confronted with two surprises: In Master Bonel’s wife, the good monk recognizes Richildis, whom he loved before he took his vows—and Master Bonel has been fatally poisoned by monk’s-hood oil from Cadfael’s stores. The sheriff is convinced that the murderer is Richildis’s son, Edwin, who hated his stepfather. But Cadfael, guided in part by his concern for a woman to whom he was once betrothed, is certain of her son’s innocence. Using his knowledge of both herbs and the human heart, Cadfael deciphers a deadly recipe for murder.
When a visitor to the abbey dies, Brother Cadfael faces a personal drama. For not only was the man poisoned by monk's hood oil, made in Cadfael's own laboratory, the dead man's widow is also the woman to whom Cadfael was betrothed before he took his vows.