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.

Mako is a magical Island that a pod of mermaids call home. Their peaceful lives are turned upside down when Zac, a merman develops a connection with the island that poses a threat to their existence. Ondina and Mimmi, two young mermaids from the Mako pod, are determined to break Zac's connection to Mako Island and reclaim Mako for the pod. As the mermaids struggle to take back his powers, they discover that his connection with Mako isn't quite an accident. New mermaids and mermen are met along the way, and some are even created!

Life is trouble-free on Mako Island – until Ondina meets Weilan, a mermaid from the Eastern pod. They immediately clash and Mimmi must act as peacemaker. But a greater threat to their peace has arrived, in the form of a fearsome Chinese water dragon that once destroyed the Eastern pod. But the dragon is not all it appears to be. On their perilous quest to defeat the dragon, Zac and the mermaids discover shocking secrets until a final confrontation reveals all.