In the waters surrounding Mako Island, real-life mermaids Sirena, Nixie, and Lyla are part of a mermaid community. As young members, their responsibility is to safeguard the Moon Pool from intruders. However, during a full moon, the playful mermaids neglect their duties, allowing 16-year-old Zac, a human, to enter the Moon Pool and establish a unique bond with Mako.

As a result, Zac gains a fish-like tail and extraordinary abilities. This forces the mermaid community to abandon Mako, leaving Sirena, Nixie, and Lyla behind, exiled from their group. To rejoin their community, the trio must transform into humans, venture onto land, and reclaim Zac’s powers, or they risk remaining outcasts forever.