ACT ONE: Outside the King's palace, the people of Ys are celebrating a coming peace treaty. Margared, one of the King's two daughters, has been engaged to an enemy prince, Karnac, and their wedding promises to end a long and bitter conflict between Ys and Karnac's kingdom.
But Margared is hardly behaving like a blissful bride. She's moody, and subdued, and her sister Rozenn wants to know what's bothering her. It turns out that Margared dreads her marriage to Karnac. She's actually in love with a local leader named Mylio, who has been forced out of the city. And there's another complication — Rozenn is also in love with Mylio. Naturally, the people around them don't know any of this, and Margared is led away to prepare for the wedding ceremony.
Alone, Rozenn ponders her love for Mylio. She's convinced that he'll soon be back, and she's right. As she's singing, Mylio suddenly appears. He has returned to Ys in secret, and the two greet each other joyfully.
Then trumpets are heard, announcing the arrival of Karnac. When Mylio learns of the proposed peace agreement, he decides that it's safe for him and his men to return home. He leaves, telling Rozenn he'll see her again that night.
When Karnac arrives, he and the King of Ys embrace, and the two confirm their new agreement. But things go sour when Margared turns up. Rozenn has told her that Mylio is back in town. Hearing that, Margared refuses to go through with the wedding, and Karnac is deeply insulted. He orders his troops back into battle, as Mylio appears with his men and vows to defend the city.
ACT TWO: From her rooms in the palace, Margared can see Karnac's troops preparing for war. She also overhears a conversation going on below. Mylio has pledged loyalty to Ys, and in return the King offers him Rozenn's hand in marriage. This sends Margared into a jealous rage. She confronts Rozenn, curses her, and vows to have revenge.
Karnac's troops attack, and scene two takes place after the battle. Mylio returns victorious. The people acclaim him as their savior, but he gives all the credit to Saint Corentin, the patron saint of Ys.
Meanwhile, though his army has been destroyed, Karnac himself is still alive. Margared seeks him out, and says there is a way he can still prevail. Ys's only protection from the sea is a single dike. Margared can lead Karnac to the sluice gates. Together, she says, they can open them, allowing the ocean to breach the dike and destroy the city. He agrees, but as they pass by the Cathedral of Saint Corentin, the Saint himself magically appears, stepping out of his tomb. He rebukes Karnac and Margared, and a heavenly choir demands their repentance.
ACT THREE: The wedding of Mylio and Rozenn is underway. While the ceremony takes place in the royal chapel, Margared and Karnac confer. Margared has had second thoughts about their plan, but Karnac persuades her to lead him to the sluice gates, and they leave together.
As the wedding party is leaving the chapel, Margared returns alone. Hiding, she overhears the King asking Rozenn why Margared is absent. Rozenn covers for her sister, saying that Margared is still a loyal and loving daughter. Hearing this, Margared is deeply moved, and immediately regrets what she has done.
But just then, there's a commotion in the distance, and roar of rushing water can be heard. Margared admits that the gates have been opened, and the ocean is bearing down on the city. Mylio also appears. He says he found Karnac at the sluice gates, and killed him, but it was too late to stop the flood.
The final scene takes place on a high cliff, overlooking Ys, where the people have fled to escape the disaster. Margared is overcome by guilt and grief. She offers herself as a sacrifice to Corentin, hoping that he'll step in and stop the flood. She runs to the top of the cliff, and leaps into the rampaging water. Saint Corentin then appears to the people, moved by Margared's death. At his command, the waters subside, saving the city of Ys from destruction, and the opera ends with a chorus of thanksgiving.
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