Rebellious 16-year-old mermaid Jack Charles is fascinated with life on land. On one of his visits to the surface, which are forbidden by his controlling father, King Triton, he falls for a human prince. Determined to be with his new love, Jack makes a dangerous deal with the sea witch Deano 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 mer-son.