When a local landowner is poisoned by a lethal concoction from the monastery's own stores, Brother Cadfael must navigate the rigid social and religious hierarchies of 12th-century Shrewsbury to clear an innocent man. Cadfael, a former crusader turned monk, utilizes his extensive knowledge of herbalism and human nature to investigate the crime. He faces opposition from local authorities who are quick to assign blame and from the complex web of family secrets surrounding the victim. The narrative, presented in a third-person perspective, emphasizes the tension between the sanctuary of the abbey and the volatile political landscape of the Anarchy period in England.

This work is widely regarded as a cornerstone of the historical mystery genre, praised for its meticulous period detail and the depth of its central character. Readers frequently note that the blend of monastic life and criminal investigation provides a unique perspective on medieval society. The narrative style is often described as thoughtful and immersive, allowing the reader to inhabit the 12th-century setting fully. While the pacing is deliberate, the strength of the character development ensures that the mystery remains engaging throughout the narrative arc.