Young mermaids of the Mako Island pod, Sirena, Lyla, and Nixie, have the task of keeping trespassers off the magical island. On a full moon, they fail to stop a land boy, Zac, from falling into the sacred Moon Pool, and the power of the moon transforms him into a merman! As punishment the mermaids are banished from their pod as it flees Mako. To redeem themselves, the mermaids must find a way to turn Zac into normal boy once more.