Part I
When the moon comes, nature seems to wake up and start living its wonderful fairy tale life. Fairies run out of the forest. Playing and singing, they lure the water spirit living there - Aquarius - from the depths of the water. He benevolently takes care of the raging youth, sometimes he himself joins in a fun game, and sometimes he tells interesting things... He also knows about the world of people, great, but full of deceit... Who could have guessed that those stories would ignite a longing in the heart of the youngest mermaid, that she will begin to observe that unknown world that lives in the sunlight...
When the fairies are silent, the Little Mermaid tells Aquarius that she is in love with the handsome Prince, who sometimes stumbles here while hunting, rests and enjoys the coolness of the lake. She began to wait for the Prince, longing for those short, happy moments when the transformed wave would lull him and caress him. She began to desire a soul and love... Aquarius, after hearing Undine's confession, gets scared, because he feels that this love of hers will not end well for her and that life among people can only bring her grief. Seeing that Mermaid does not listen to his warnings, he advises her to turn to the Witch.
Left alone, Mermaid asks the moon to let her beloved know about her love with its rays. After the distant voice of Aquarius tries again in vain to dissuade the Mermaid from her plan, she calls out to the Witch. She says she knows a way to turn a water fairy into a human being, but warns that it is dangerous: after becoming a human, Mermaid will lose her speech, remain mute, and if she disappoints her love, she will be cursed, and then she will not find peace either on earth or in water. And besides, the one she loves will also die. Convinced of the power of her love, the Mermaid agrees to these terms, and the Witch transforms the Mermaid into a human with her spell.
It's morning, hunting horns are sounding. A hunter's song is heard. The prince chases a white stag. As in a dream, he can't move from his place, not even the hunter's song wakes him up. Instead of a doe, he sees a beautiful girl sleeping in the grass. In vain, Mermaid's friends and Aquarius from the depths of the water try to stop her. The prince, suddenly in love with Mermaid, takes her to his castle.
A big celebration in the prince's castle is the engagement of the young master. Everyone is worried - the Prince has chosen a strange bride! He thought of marrying a mute found in the forest, maybe a woman, or maybe a fairy... The forester is talking to the kitchen assistant, the cook, both of them are surprised by the frightening impression that the Mermaid makes on them. And the Prince is worried. They have been living side by side for so many days, and he is unable to solve the mystery of this mute creature, to ignite a fiery passion in her heart... Disappointed in Undine, he directs his feelings to a foreign Princess who appears in the garden. No, the proud Princess doesn't need the Prince's love, but why not enjoy another victory?
Suddenly the garden lights up, a merry fanfare is heard and a group of flamboyant guests leave the castle. The guests dance the polonaise. After the guests disperse around the garden, Aquarius emerges from the pond. He has already noticed that the Prince is about to lose the Mermaid, and right there he mourns the sad fate of the Mermaid. His cry joins the chorus of guests returning from the garden.
The Mermaid appears in the empty garden. Seeing Aquarius in the pond, she tells him about her grief. The Prince and Princess enjoying each other nearby are living proof of her grief. The Prince declares to the Princess that he has already forgotten Mermaid because of her, and after both heated confessions, the Prince squeezes the Princess in his arms. The Mermaid watches this dangerous game with horror, she tries to pull her lover away from the charming sleigh, but now the Mermaid's touch has taken him away with the cold! The prince pushes her away, Aquarius, emerging from the pond, embraces the Mermaid and dives into the water with her. In vain, the frightened Prince asks the Princess to save him from the enchantment. The princess for whom he lost Mermaid rejects him and proudly walks away.
 
Part II
Broken, the Mermaid stands by her native lake. Death would be salvation for her, but mermaids don't die... The witch's curse turned her into a serpent. The witch sees her grief and offers a way out: she must kill the impostor with her own hands. Only his blood can wash away the curse. But Mermaid's love is greater than suffering - she cannot kill her beloved, she takes the entire burden of the curse on herself...
The forester and the cook also come to consult with the Witch. They would like her advice on how to cure the Prince under the Mermaid's spell. However, they are not successful. Aquarius, hearing their speech, swears to take revenge on the human race.
At dusk, fairies gather in the meadow to play. They are happy and laughing, but Aquarius does not approve of their fun. He tells them about the terrible fate of their friend, Mermaid, and they hide in the forest, saddened.
The Prince appears. He runs like crazy to the lake. Tormented by remorse and guilt that he realized too late that he loves only her, he searches everywhere for his white doe, his Mermaid. Undinė, who appears to him, bitterly reproaches him for his infidelity and says that now her kiss will only bring him death. The prince is not afraid of death, he falls into Mermaid's arms and dies happy.