Rebellious 18-year-old mermaid Danielle is fascinated with life on land. On one of her visits to the surface, which are forbidden by her controlling father, King Andrew, she falls for a human princess. Determined to be with her new love, Danielle makes a dangerous deal with the sea witch Ursula to become human for three days. But when plans go awry for the star-crossed lovers, the king must make the ultimate sacrifice for his daughter.