Sirena, Nixie and Lyla are part of a mermaid pod, which lives in the waters of Mako Island. It is their job to protect the Moon Pool and guard it from trespassers. But on the night of a full moon, the mischievous mermaid girls neglect their duties. Sixteen-year-old land-dweller Zac enters the Moon Pool and is given a fish-like tail and amazing powers.