First light, Ascension morning. From the top of the tower at the College of the Transfiguration, voices rise in song. Felicity's delight turns to horror when a black-robed body hurtles over the precipice and lands at her feet, the strange emblem of a double-headed snake clutched in his hand. Matters grow murkier as Felicity and Antony, leading a youth pilgrimage through rural Wales, encounter the same sinister symbol as they travel. Lurking figures follow them through the land of dragons and old Celtic saints. Then a body is found face-down in a well …