It revolves around Merliah Summers, a surfing champion who learns she is half-mermaid and sets out for an undersea adventure to rescue her mother, the queen of an ocean kingdom.

Merliah and her dolphin friend Zuma set off on an undersea adventure to rescue her mother, the queen of Oceana.

While participating in a competition, Merliah loses concentration when her hair spontaneously gains pink streaks, causing her to wipe out.

While underwater, she discovers that she is able to breathe, and is approached by a talking, pink bottlenose dolphin named Zuma.

After telling her friends Fallon and Hadley the story, Zuma appears to the girls, confirming that Merliah is half-mermaid.

The Dreamfish offers to grant Merliah's deepest wish and send her back to Malibu and erase her mermaid half for good.

Merliah rejects the wish and accepts her responsibility as the princess of Oceana, gaining a real mermaid tail which she uses to escape the whirlpool herself.

Calissa is made queen once again, and, now able to spin Merillia at full strength, restores Oceana to its former glory.