In Hans Christian Andersen's "The Little Mermaid," the title character is motivated, out of love for the prince and out of a desire to attain an immortal soul (which mermaids do not possess), to make a deal with the sea witch to attain human legs so as to coexist with humans. As payment, the sea witch takes her tongue, removing her ability to speak or sing. Should the prince marry her, she would stand to receive an immortal soul, but should the prince marry someone else, then she would turn into sea foam.
Ultimately, in Andersen's tale, the mermaid's romantic hopes end in disappointment. While the prince has great affection for the mermaid, he has no desire to marry her, and by the end of the story, he is set to marry another. This brings about the climax of the tale, by which, after all her agony and torment, the mermaid finds herself face-to-face with her own mortality, aware that she will turn to sea foam with the coming morning.
It is in this context that the sisters return to her, having made their own deal with the sea witch, bartering their hair for a chance to save the mermaid's life. They present her with a knife, and tell her that if she were to drive the knife into the prince's heart, killing him, then she would return to her life as a mermaid. This is the dilemma that stands at the center of the story and serves as its key scene: she can kill the prince to save herself, or she can spare the prince, which means an immediate death with no hope of salvation. She tosses the knife away, surrendering to that fate.
The story ends on a hopeful note, however, as because of her suffering and sacrifice, she is transformed once again, not into a mermaid but into one of the "daughters of the air," who are given the chance to earn their souls through good deeds and service to the world.