Season One: The lifes of three girls living on the Australian Gold Coast are changed forever. When Cleo, Emma and Rikki stumble upon a pool inside the dreaded Mako Island, a full moon transforms them and the next day they start turning into super powered mermaids! With the help of friend Lewis, the girls manage to control their powers and hide their secrets. But it becomes difficult with maddening full moons, nosey scientists and local bad boy Zane going on a quest to find them! Season Two: Following the incident with Dr Denman and the Lunar Eclipse, Emma, Cleo and Rikki want to go back to their normal lives. But when the full moon comes back, all three girls have a dramatic power change! Without Lewis, they could destroy themselves. But their friend is trying his best to help them, despite his relationship with geeky Charlotte. But Charlotte proves to be more of a threat when she learns about her grandmother being a mermaid and stumbles into the moon pool and becomes a mermaid herself!
Cleo has to put off Lewis' romance plans in order to help Kim with her crazy love life. Meanwhile, Don meets up with a National Parks Manager, Samantha, and the two decide to date.
With Will finding out about Mako Island, Bella tries to misdirect his search so that he will not discover the Moon Pool. However, Lewis and the girls find out that the Moon Pool is different.
To get more interest in the cafe, Zane develops a dirt bike race. However, Rikki finds out that it is a scam. She and Bella decide to team up to teach him a lesson. Meanwhile, Lewis and Cleo find out something interesting about mixing the waters in the Moon Pool
Cleo's 16th birthday is approaching and she thinks that her father is going to throw another embarrasing birthday party. Meanwhile, Zane tries to find the 'sea monster' that he saw when he almost drowned.
Sophie uses Bella's idea for a fundraising concert and presents it as her own to Rikki to gain publicity for the cafe. Cleo's caught in the crossfire with Kim when Samantha moves in and puts her stamp on the Sertori home.
At the marine park, Cleo seeks a job as a dolphin trainer. Meanwhile, until she and Bella reach an understanding, Rikki fires Nate's band, preventing them from playing in the cafe.
Cleo and Rikki enter their final year of high school. They make friends with a new girl, Bella, and a new boy, Will. However, they encounter a new force on Mako Island that has the power to turn water against the girls.
Rumour has it that Cleo has a boyfriend and her friends and family are trying to find out who he is. Meanwhile, Zane continues his search for the 'sea monster' he saw.
Cleo and Kim are stuck with their cousin Angela who wants to go body boarding and causes a bird brained distraction when Cleo refuses.
Trouble is in the air as the Setori's start to suffer and the JuiceNet is endangered by a new bar. But when Lewis finds a book of mermaid spells, the girls attempt to use a potion to fix everything. However, things aren't so easy when you have special powers.
Rikki is offered a job to collect rare fish. At first it seems perfectly fine, until she finds out the person that hired her is selling the fish for large profits.
Rikki enters a short film contest on the theme of 'Heroes' and decides to capture some nice underwater footage with her 'abilities'. Meanwhile, Emma is interviewing her mum and Zane and Lewis are planning to rennact Dr. Bennet's windsurfing record.
Cleo decides to restart her friendship with Lewis and embrace her abilities, while Emma is shocked with the new supervisor who has big changes planned.
Emma invites Miss Chathams to stay at her place and Zane overhears Miss Chathams concerning about 'her treasure'.
Miss Chatham gives Cleo a warning that she doesn't quite understand. Until Emma finds herself in a trance and unable to change back from a mermaid under a full moon.
Annoying Miss Popularity, Miriam, is having a pool party and the girls want to go. But being around water is risky, especially when Zane tosses Cleo into the pool. Lewis is the only one around to help so he finds out their secret.
Rikki tries to avoid Zane when he comes back, but needs his help after an incident with a speedboat. Meanwhile, Charlotte continues to flirt with Lewis and Cleo worries for their secret.
The full moon gives Cleo a power to put guys in a 'trance' and she suddenly gets a rush of admirers.
Emma is doing well at her new job at the JuiceNet cafe, but when she is left in charge, she has trouble managing her new employee, Rikki.
The girls discover the Mako crystals contain immense energy and need to mount a rescue mission when Zane jealously steals one in an effort to gain Rikki‟s attention.