In the Fifth Century, the World is changing. A conflict has begun between Christianity and the ancient religions. In the Shade, druids and monks are engaged in a war. As the Bretons occupy the rocky headland of Armorique, Gwenc' hlan, the last druid of this region, and Taran, his apprentice, try to thwart the traps set by their fanatical opponents. To succeed, they will need the help of Morrigane, a woman of the Other World...

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.

There is turmoil in the 5th century Celtic periphery of Europe; an age of change and an age of conflict between peoples and religions. Oppressed by Saxons to the east, many of the Celts of Britain have colonised Armorica in Gaul, bringing with them the spiritual leaders of their Celtic Church, the monks. The new faith heralds the demise of the old pagan way of life, but the ancient traditions still hold on. Gwynlan, the last of the Druids, is caught in a mysterious battle for survival against the monks of the church, monastic foes who place dangerous and menacing obstacles in their path. Accusations of murder, sacrifice and sorcery are rife, but the old gods are at hand to assist the righteous.