Zac is a teenage boy who decides to camp on Mako Island, unaware that three mermaids, Sirena, Nixie, and Lyla, who are guardians of the island, are watching him. That night, when the full moon rises, the boy comes into contact with the magic water of the Moon Pool. The following morning, Zac discovers that he has the ability to control and manipulate water. Later, after accidentally falling into the water, he finds that he has also become a merman with a fish-like tail. His new-found merman abilities will cause problems for the mermaids. After their pod casts them out for allowing Zac to become a merman, the three mermaids, curious about living on land and motivated to remove Zac's powers, venture onto the land and learn to live among humans.