Real-life mermaids, Sirena, Nixie and Lyla are part of a mermaid pod, which lives in the waters of Mako Island. As young members of the pod, 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 forms a special connection with Mako.Zac is given a fish-like tail and amazing powers. The mermaid pod is forced to leave Mako, leaving behind the three mermaid girls, cast out of the pod. They know there's only one way they will be allowed to rejoin the pod: They must get legs, venture onto land and take back Zac's powers – or risk being outcasts forever.

H2O: Just Add Water revolves around three teenage girls facing everyday teen problems with an added twist: they cope with the burden of growing a giant fin and transforming into mermaids whenever they come in contact with water.