Near a lake, three water sprites dance in the moonlight. Rusalka sits sadly by the water and laments to her father, Vodník, that she is in love with a human – the Prince. She met the prince when he came to swim in the lake and instantly fell in love. She now wants to be with him as a human. Although Vodník attempts to dissuade her, Rusalka insists and calls on the witch, Ježibaba, to help. Ježibaba agrees to create a potion that will turn Rusalka into a human, but notes that the transformation comes with conditions. Once human, Rusalka will lose her voice, and if she doesn’t find love, both she and the prince will be eternally damned. Rusalka agrees and drinks the potion. The Prince, hunting in the forest, encounters Rusalka, embraces her, and leads her off to his castle. Vodník and Rusalka’s sisters mourn her departure.
At the Prince’s castle, the Prince and Rusalka enter and he expresses his frustration about how cold Rusalka has been. The Foreign Princess, who has arrived for the wedding, catches the Prince’s eye. The Prince sends Rusalka away so he can accompany The Foreign Princess to mingle with wedding guests.
In the garden, Vodník appears in the lake. Rusalka runs to him and suddenly recovers her voice. She begs for his help, telling him that the Prince has rejected her. The Prince and The Foreign Princess enter, and the Prince confesses his love for The Foreign Princess. Rusalka runs to the Prince, imploring him to return to her, but he rejects her yet again. Vodník warns the Prince of his fate, and disappears into the water with Rusalka. The Prince asks The Foreign Princess for help, but she tells him to follow his love to hell.
Back at the lake, Rusalka mourns her fate to Ježibaba. Ježibaba says that she can avoid her fate by killing the Prince. But Rusalka refuses, and sinks into the lake where she is also rejected by her sisters. When Vodník tells the three water sprites what has happened to Rusalka, they stop dancing and fall silent.
The Prince enters, and asks Rusalka to return to the castle with him. She refuses and explains that if she kisses him now, he will die. The Prince begs her to kiss him and give him peace. She does, and he dies in her arms. Rusalka asks for mercy on his soul and disappears into the water.