Real-life mermaids Sirena, Nixie and Lyla are part of a mermaid pod, living in the waters off Mako Island. Their job is 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. Suddenly sixteen-year-old land-dweller Zac enters the Moon Pool and is given a fish-like tail and amazing powers. The girls must venture onto land and take back Zac‘s powers - or risk being cast out forever!