The marriage of Hugo and Undine (adoptive daughter of a fisherman) is attended (in disguise) by Undine's real father, the water prince Kühleborn.
He knows that Hugo should not have married Undine, because he was promised to Berthalda, the daughter of Duke Heinrich. Concerned for his daughter's wellbeing, Kühleborn follows the couple back to Hugo's home.
Undine reveals her true identity to Hugo. Berthalda seems unaffected by the news of Hugo's marriage and informs everyone of her engagement to the king of Naples.
Kühleborn discloses that Berthalda is in fact the daughter of the fisherman and then discloses his own identity.
Undine warms to Berthalda and invites her to live with her and Hugo. Berthalda seduces Hugo, and Undine is carried back into the waters by Kühleborn.
As Hugo prepares to marry Berthalda, Undine appears, filling the great hall with water. She
drags Hugo into the depths, where Kühleborn forgives him - but only if he remains by his daughter's side in the palace of the water spirits.