The source of evil and the conflict between faith and knowledge are among the themes examined in this complex intellectual puzzle set in a medieval monastery. An Irish monk dies in a Benedictine abbey in northern Italy on the eve of the second millennium. When Brother Thomas of Worms attempts to investigate the murder, his abbot accuses him of inventing trouble to avoid his duty to God and assigns Thomas the chore of copying a Biblical text as penance. Neither copying nor humility comes easily to an intelligent man like Thomas, who struggles with his commitment to obey his abbot. While in the library, Thomas is drawn to a gnostic book that leads him to a discovery that threatens the very fabric of the Church. When more monks perish, Thomas's loyalty to the monastery and its rites is tested, and he risks expulsion as he seeks to uncover the link between the murders and the hidden codex that has shaken his faith.