Felicity's ecstatic enjoyment of her fellow ordinands' singing in Ascension Morn from the top of Pusey Tower at the College of the Transfiguration is shattered when a black-robed body hurtles over the precipice and rolls to a stop at her feet. Father Antony recognizes the corpse as Hwyl Pendry, a former student of his, who has been serving as Deliverance Minister in the diocese of St. David's. The police ignore the strange emblem of a double-headed snake curling across a triangle Hwyl clutched in his hand and label the death a suicide. But Hwyl’s widow is convinced otherwise, and pleads for Felicity and Antony to help her uncover the truth. Matters grow murkier as Felicity and Antony, leading a youth pilgrimage through rural Wales, encounter the same sinister symbol of a double-headed snake on their travels. Lurking figures follow the group. Then one of the young people is found face-down in a well …