Undine rescues a prince from drowning, and leaves him unconscious on shore, where he can be found, but she has already fallen in love with him.  Unable to enjoy life below the waves, the mermaid makes a deal with the Sea Witch to become human.  The mermaid must also give her voice as payment. And her humanity is only conditional: if she can make the prince fall in love with her and marry her, she can gain a share in his soul and be truly human, but if he marries another, she will die at the next sunrise.  It turns out that the witch only made her believe she would die if the prince married another as a Secret Test of Character: by refusing to kill the prince even to save her own life, she proves that she already has a soul. So she remains human, gets her voice back, and sets out to explore the world.