As part of a peace agreement, Margared, the daughter of the King of Ys, is bethrothed to Prince Karnac, a former enemy of the city. During the celebrations she confesses to her sister Rozenn that she really loves Mylio, a childhood friend who had sailed away years before. Margared is convinced that Mylio returns her love. During her wedding ceremony she learns that Mylio has unexpectedly returned and refuses to go ahead with the marriage. Karnac curses Ys and threatens vengeance.
Margared discovers that Mylio actually loves Rozenn and overhears the King promising Rozenn's hand to Mylio when he returns victorious from combat with Karnac. She is overcome with jealousy. Mylio indeed returns victorious, attributing his success to the support of the city's patron saint, St Corentin. On seeing the defeated Karnac, Margared offers to join him in seeking revenge. The statue of St Corentin warns her to repent, but she ignores him and plans to give Karnac the keys to the sluice gates that protect the city from the sea.
During the wedding ceremony of Mylio and Rozenn, Margared's resolve begins to waver. However, Karnac re-ignites her jealousy and desire for revenge, and they head for the sluices. The King notices Maragred's absence from the ceremony and is troubled. Margared returns and announces to all that Ys is doomed - Karnac has opened the sluices. Mylio kills Karnac but too late to save the city which is now being engulfed by waves. Half of its citizens are drowned and the remainder are terrified. Margared, stricken with remorse, tells them that the ocean demands a sacrifice and hurls herself into the sea from a high rock. Upon her death, St Corentin appears and calms the waves thus saving the city.