Gradlon reigned formerly on Cornwall surrounded by two saints who advised it, the Guénolé monk and the Corentin hermit of which he had made his bishop by giving him his castle of Quimper. The king had a beautiful girl, Dahut , for whom he had made build a splendid city: the town of Ys. This city, located on bay of Douarnenez was protected from the sea by dams and locks .The king kept the key. Dahut carried out a life of idleness and pleasures. One day, on the dam, she meets a tempting knight who asks her to conceal the keys that her father preserve around the neck. Benefitting from the sleep of the king, she steels the keys and gives them to the red knight who is not other than the Devil… This one opens the locks and the city is invaded by the floods. Only Gradlon, assembled on his horse Morvac' H and Guénolé succeed in escaping. Drowned, Dahut is changed, says one, in a siren, Morgane, which is always baited to lose the sailors…