Deep in the dark forest is a lake where strange creatures revel and frolic mischievously, but one of their number, Rusalka, the water nymph, is gripped with yearning. She is in love with a prince, who is drawn to her waters at night, but he cannot see her: she is a prisoner of her element. The Water-Spirit, who has just been cavorting with three Wood-Sprites, listens with horror to Rusalka’s desire for a human body and soul – he tells her these things are full of sin, and not something for which she should yearn. Rusalka, however, is firm in her resolve, and the Water-Spirit, with great foreboding, sends her to Ježibaba the witch. Ježibaba lives close by, and she gives Rusalka the power to leave the water, hobbling on limping feet over the ground. Full of scorn, she listens to the water nymph’s wish, and gives her the thing she desires: a human soul. In exchange, Rusalka must leave her voice behind, and listen to the curse of the elements: if she is unable to keep her prince’s love, she will be damned to an eternity caught between life and death, and will drag her beloved with her to his doom. A huntsman’s voice is heard in the forest. Following close behind him is the Prince, who for weeks has been hunting a white doe which continually eludes him. Arriving at the lake, a place full of familiar and mysterious enchantment, he sends his retinue back to the palace in order to be alone. There he encounters Rusalka, now mute, whom he perceives by turns as his white doe, and as a creature of unearthly beauty from a fable. From the depths of the lake, the alarmed cries of Rusalka’s sisters ring out: one of them is missing! Not waiting for an answer to his questions of who or what she is, the Prince takes Rusalka to his palace.
At the Prince’s palace, a great feast is being prepared. The Gamekeeper and the Kitchen Boy are gossiping about the mysterious, cool beauty whom the Prince has brought from the forest, and who has swiftly changed his character for the worse. The Prince asks Rusalka over and over what her nature is, but receives no answer. He wishes to possess her completely, but feels an inner resistance. Out of arrogance and spite, a Foreign Princess resolves to divide the couple. She mocks Rusalka, and rebukes the Prince, pressuring him to entertain her.
The Prince sends Rusalka away to prepare for the celebrations, and sets off arm in arm with the Princess for a stroll. The guests arrive, and Rusalka does not know where to turn. The Water-Spirit appears, lamenting Rusalka’s path to ruin: she will never find what she yearns for in the human world, and she can never return to their watery realm. Rusalka rushes to him, speaking for the first time in weeks. The Prince’s love is slipping away from her, because he is seeking human passion, which she does not have. Rusalka and the Water-Spirit look on helplessly as the prince ardently declares his love to the Foreign Princess. When Rusalka throws herself into his arms, he pushes her away with a shudder. The Foreign Princess celebrates her triumph over them both, condemning the Prince and his beloved to hell.
Now under the curse of the elements, Rusalka must live as a will o’ the wisp, luring strangers to their doom. Ježibaba has nothing but scorn and contempt for the water nymph. Her only hope is to wash herself clean of the curse with the prince’s blood, but Rusalka is horrified at the thought of such a barbarous act, and she throws Ježibaba’s knife far out into the lake. Cast out by her kind, she is now caught between worlds, neither alive nor dead. The Gamekeeper and Kitchen Boy come to Ježibaba, pleading for help for their Prince, who is sinking ever deeper into crazed despair. But Ježibaba wouldn’t dream of coming to the aid of these men, and the Water-Spirit suddenly appears, driving them out of the forest with his threats. The Wood-Sprites we saw earlier now appear at the shore, admiring their own charms in the moonlight. When they begin their habitual teasing of the Water-Spirit, he tells them the sad tidings of Rusalka’s fate, at which the three sprites flee in alarm. Suddenly, the Prince appears at the lake shore, calling for his white doe. Rusalka appears hovering above the lake, and warns the Prince he will find nothing but death in her arms. However, his yearning for her is so strong, the Prince compels Rusalka to give him one final, fatal kiss. Left entirely alone, Rusalka commends the Prince’s soul to God’s forgiveness, and goes to her eternal doom.