Gwynlan and Taran venture with their companion monk Budog to the legendary city of Ker-Is. Built at the edge of the sea, with mighty walls as protection from the tides, Ker-Is is governed by princess Dahud, daughter of King Gradlon. Although Dahud remains faithful to the old Celtic religion she is willing to welcome monks of the new Celtic Church into her city, to tempt them with sinful pleasures. Despite her sensual appearance, Dahud is extremely strong-willed, defying all who attempt to seek her hand in marriage. However when her latest suitor arrives at the gates of Ker-Is, so does new evidence to help Gwynlan unravel his murder mystery.