Lewis find out that there is a way for a mermaid to lose her powers and attempts to lure Charlotte to the Moon Pool. The plan backfire and it looks like all the girls may lose their powers.
Charlotte's short-lived friendship with the other girls is over and she demands that out of loyalty to her, Lewis have no further contact with Rikki, Emma and most of all, Cleo.
Zane is curious by Rikki's reluctancy to reveal where she lives. So one day, he eventually convinces her to reveal her home: a trailer park with her grouchy father.
Lewis is trying to find more information on the moon pool and stumbles across research on it by Max Hamilton from fifty years ago. Will Lewis reveal the girls' secret to him in order to complete the puzzle?
In need of cash, Rikki decides to go and searching for a lost statue with a huge reward. But she lies to Emma and Cleo, which causes danger when Rikki is knocked out.
Rikki's frustrated by the water attacks and goes to Mako Island on a full moon to confront the strange force in the Moon Pool. With Cleo and Bella delayed, Will pursues her and discovers she's been affected by the experience.
The girls prepare for the full moon again and insist they don't need Lewis' help. Rikki and Cleo suggest Ash to be more bold with Emma. Following their advice, Ash decides to do a surprise visit which could uncover their secret.
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.
The girls sit their final school exams and have to avoid a water attack in public when the full moon rises early. They confront the water force in the Moon Pool and are given a message about Mako Island.