King Sigismund orders his son Nikolas to finally get married and become a proper heir to the throne. Nikolas, failing to fall in love of an even properly date, the king arranges a politically suitable bride, spinster princess Anneline. Enjoying his last 'free man days' incognito, gallivanting with his trusted squires, athletic Nikolas goes swimming but nearly drowns. When he beaches up, Anneline falls in love with the handsome stranger but rides on to court. Nikolas's party tries to thank the mystery woman who rescued him, ignoring its sea god Neptune's rebellious daughter, mermaid Undine, who has such a bad crush that she accepts the sea witch Mydra's deal to change her tail into legs but render her deaf and end her life unless she finds true love. She becomes Nikolas's favorite playmate, but he seems determined to do his dynastic duty by wedding princely.
