Gervase Bonel, his wife and his servants are guests at the Shrewsbury Abbey of Saint Peter and Saint Paul. They have bequeathed their manor home to the monastery in exchange for care and lodging during the rest of their lives. Brother Cadfael, the monastery's resident herbalist, is urgently called upon one evening to rush to Bonel's bedside. Bonel has been fatally poisoned by one of Cadfael's own herbal remedies, monk's-hood, which has been mysteriously added to his dinner plate. In this enchanting medieval tale of mystery and intrigue, Cadfael, driven by his own personal reasons, sets out to vindicate Bonel's stepson Edwin, the Sheriff's primary suspect. Drawing upon his knowledge of both herbs and the human heart Cadfael ultimately uncovers a deadly recipe for murder.