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.
Two young mermaids, Ondina and Mimmi, run away from the Mako Island pod with Sirena to try to turn the merman Zac back into a land boy. But Zac’s connection to Mako is growing stronger. If they fail, they may never break his connection and save Mako for the pod. On the full moon, they attempt to remove his merman powers when his girlfriend Evie comes to the rescue – only to be turned into a mermaid!
Cam is on lifeguard duty when Erik quizzes him about Zac's connection to Mako. Erik is splashed and rushes into the ocean, disappearing. Cam paddles out in search of Erik, only to discover that Erik is a merman. Meanwhile, Sirena and Ondina engage in a contest of mermaid powers, whipping up a dangerous storm over the ocean. Cam gets in trouble for losing Erik, but Erik stages a fake rescue, making Cam a hero. Erik now has an ally in Cam – and Zac doesn’t like it.
Mimmi and Sirena prepare a potion that helps the user to look younger. Rita applies some before a school excursion to the Marine Park, accompanied by Dr. Ross, the park's handsome Director. Rita is unaware that Ondina accidentally altered the potion, which now makes her act younger as well. Evie and Zac have to move fast to stop the excursion turning into a disaster, while Rita behaves like a young child, threatening to reveal the truth about mermaids when she decides to go for a swim.