The real-life mermaids, Sirena, Nixie, and Lyla, are part of a group of mermaids that live in the waters of Mako Island. As young members of the group, it is their duty to protect the Moon Pool and defend it from intruders. But on the night of a full moon, the playful mermaid girls neglect their responsibilities. Zac, a sixteen-year-old boy from the land, enters the Moon Pool and forms a special bond with Mako. Zac receives a fish-like tail and amazing powers. The group of mermaids is forced to leave Mako, leaving behind the three mermaid girls, excluded from the group. They know that there is only one way they will be accepted back into the group: they must obtain legs, venture onto land, and retrieve Zac's powers – otherwise, they risk being excluded forever.