The winegrower Josef Fink has his eye on the vineyard of his neighbor Franziska Habicht. When fate hands him an ancient contract stating that the Habichts' vineyard became the property of the Finks back in 1602, a small war begins between the winegrowers. Naturally, Franziska leaves no stone unturned to contest the document. In the process, her son David, who is in love with Josef's daughter Marie, uncovers a family secret.

Once before, the ravishing lake mermaid Undine has warned her lover Raoul. Their humid-romantic fairy-tale love is in danger because Raoul's brother wants to destroy the pristine mountain lake idyll with an ambitious hotel project. Undine defends her paradise. She is alluring, but also perverse and unpredictable. She can give a lot of love, but she can also kill.