Druid Gwynlan's investigation into the heinous murder of Brythonic monks takes him across the sea to Britain. Two of the Treasures of the Britons, the Cauldron of Rebirth and the Spear of Lug, are coveted by the Church of Rome, hell bent on eradicating the old druidic faith. Aided along a safe but circuitous path by the seer Myrddin, Gwynlan and his retinue reach the Giant's Dance – Stonehenge – whose ancient stones conceal part of the secret which puzzles Gwynlan. A strange revelation takes him into a deadly swamp where Taran's life faces mortal danger. Gwynlan's apprentice can only be healed on the druids' sacred island, Môn.