The magnificent city of Ys in Brittany is protected from flooding by a dike, overseen by the royal family. To seal peace with the city’s former adversary, the King of Ys grants him the hand of his eldest daughter, Margared. But she breaks her vows at the altar, sparking the outbreak of a new war. Devastated to learn that the man she wishes to marry is promised to her own sister, Margared prepares to betray the secrets of the city of Ys to the enemy.