The Little Mermaid was the youngest daughter of the Mer-King, her widowed father. She lived in the palace under the sea with the rest of her family and has a special singing ability. In the age of 15, a mermaid can go up to the surface. The Little Mermaid, being the youngest daughter, had to wait few years until the day when she would be allowed to go up to the surface.
On her fifteenth birthday, the Little Mermaid went up to the surface and rescued a prince from drowning. She hid nearby as the prince gained consciousness, and then a maiden went to aid the prince. The prince, who though the maiden saved his life, thanked her for saving his life, not knowing that the Little Mermaid is actually the one who saved his life. The Little Mermaid was saddened that the prince will never know that she saved him.
In order to be given a chance to marry the prince, the Little Mermaid decided to give up her life and identity to gain a human soul and some time alongside the prince. But she knew that as long as she is still a mermaid, she would never be able to win the prince's love. So she decided to go to the Sea Witch to help her. The Sea Witch agreed to make for the Little Mermaid a potion which will turn the Little Mermaid's tail into human legs so she would look like any other human and that every step will hurt as if she is walking on sharp knives, and her feet will bleed terribly. Also, the Sea Witch told the Little Mermaid to sell the latter's voice in order for the latter to become a human, meaning that the Little Mermaid will be mute. Furthermore, the Sea Witch told the Little Mermaid that if the prince will fall in love with her and will marry her, she would become a human, but if he will marry another woman, she will die and she will become a sea foam. The Little Mermaid, longing for a life with the prince and an eternal soul, agreed, and the Sea Witch made the potion and cut the Little Mermaid's tongue, making the latter mute.
Then, the now-mute Little Mermaid went up to the surface and drank the potion. As a result, she felt a sharp pain and she soon fainted. When she woke up, she found the prince bending forward above her and found out that her tail was transformed into human legs. The prince asked her who she is, but she could not answer him as she was mute. The prince, feeling pity for the Little Mermaid, took her to his palace anyways.
The time when the Little Mermaid was living as a human on the surface with the prince was a hard time for her, as she suffered from sharp pains like a knife, but her time with the prince was a compensation for her. The Little Mermaid became the prince's favorite companion. Once the prince told the Little Mermaid that the maiden is his true love, but he could not marry her because she is a nun who lives in the temple. He also tells the Little Mermaid that there is a princess from another region who will marry him, but he does not love her, and he will marry the Little Mermaid instead. The Little Mermaid was happy that she is going to marry the prince and become a human.
However, when the princess arrived, she was revealed to be the maiden who the prince thought she saved his life. The prince declared his love for her, and the royal wedding was announced at once. The Little Mermaid, on the other hand, was heartbroken, realizing that her fate was decreed. The Little Mermaid attended the prince and the princess's wedding, and later in the night, before dawn, the Little Mermaid saw her older sisters, and one of them gave the Little Mermaid a knife which they received from the Sea Witch in exchange for their long, beautiful hair. They told the Little Mermaid that if she will kill the prince let his blood drip on her feet, she would be a mermaid once more and would save herself from her death.
However, when the Little Mermaid entered the prince's room and found him lying with his new bride, she could not kill the prince she loved. So she decided to throw herself and the knife into the sea at dawn and become a sea foam. At dawn, the Little Mermaid threw herself and the knife into the sea and she was about to become a sea foam. Her body dissolves into foam, but instead of ceasing to exist, she felt the warm sun and discovered that she has turned into a luminous and ethereal earthbound spirit, a daughter of the air. As she ascends into the atmosphere, she is greeted by other daughters of the air who tell her she has become like them because she strove with all her heart to obtain an immortal soul. Because of her selflessness, she is given the chance to earn her own soul by doing good deeds to mankind for 300 years and will one day rise up into the Kingdom of God.
- The Little Mermaid in the original novel dissolves into foam in the end. However, Ariel, her Disney counterpart, not only that she does not dissolve into foam, but she also married the prince and lived happily ever after.
- The original story of The Little Mermaid had a sad ending, as the protagonist died. However, Walt Disney's company, knowing that the original story would not be suitable for children, changed the story so it would have a happy ending, as seen in the end of Disney's The Little Mermaid when Ariel marries Eric.