With Cleo's parents now divorced, Cleo is ordered by her dad to take more care of Kim, starting with taking her to a concert. But on the night, Cleo, Rikki and Emma all accidentally see the full moon, and take off to Mako Island, with Lewis in hot pursuit. Once there, the girls undergo another magical transformation in the Moon Pool, as a rare planetary alignment intensifies the power of the full moon – Emma's power is enhanced so that she can produce snow and frost, Rikki can conjure lightning, and Cleo can now control wind as well as water. After the moonstruck girls nearly harm Lewis, they realise that they must work to control their new powers, before anyone else gets hurt.
The girls have problems with their new powers and Cleo finds that Lewis isn't helping. A new girl called Charlotte arrives and develops a crush on a certain charecter
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.
Trouble arises when Emma throws a party to prove she is as fun as Rikki. Meanwhile, Lewis is trying to prove his manhood and Cleo is recieving cards from a secret admirer
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.
Cleo is worried when she sees her dad around with another woman. When she arrives at their house, she turns out to be Charlotte's mother! Meanwhile, Rikki and Emma attempt to save the Sertori's dinner with their powers.
Everyone is shocked when Cleo is fired at the Ice Cream Stand, and even more so when Lewis takes her place! Rikki and Emma strike back at him but end up going too far
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.
Emma has to take Eliot for horse riding lessons and the competitive perfectionist tries to be better than the instructor. Meanwhile, Lewis double dates with Charlotte and Cleo.
Cleo starts to struggle at school and gets help from the new tutor. But there are bigger problems when Charlotte starts to get closer to Mako Island with the help of Lewis.
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!
Emma gets poisioned after cutting her finger on some coral and she begins to turn into a white scaled monster! Meanwhile Charlotte thinks Cleo is trying to break her and Lewis apart.
On the night of a full moon, Lewis, Emma, Rikki and Charlotte accompany Cleo's family for a camping trip at Mako. When all three girls are affected by the moon, they are unable to stop the curious Charlotte...
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.
Zane learns of the myth of Ambergris, a fragrance that can make mermaids fall for any man that wears it. When he buys it, Nate ends up wearing it and Emma, Cleo and Rikki all fall for him!
Lewis is puzzled when he dreams about Cleo being photographed by people. Later, he and Cleo take Eliot and Kim to the marine park. Meanwhile, Emma thinks something is up between Ash and Rikki.
It's full moon and the girls insist that they don't need Lewis to assit his help as all three girls are staying at Emma's and Ash is coming over. When Cleo and Rikki fall under the moon spell, can Lewis stop them from revealing their secret to Ash?
For the first anniversary of the girls being mermaids, Cleo decides to throw a party. However, Emma and Rikki fall out following a fight between Ash and Zane and Cleo may have to celebrate alone.
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?
Charlotte demands answers to why Lewis is hanging out with the girls so much. She starts to believe an excuse from Cleo, but then a discovery from inside Max's research brings her to the secret! Meanwhile, Ash decides to help Emma with her "fear of water"
Charlotte's stubborn nature and thirst for knowledge leads her to Max, who confirms her suspicions and leads her to the Moon Pool. Lewis meanwhile is having trouble trying to keep their relationship together.
The girls decide that they should accept Charlotte, and the four work together when they need to join the volleyball team to avoid failing Physical Education.
Charlotte is determined to prove her love for Lewis and show she is better than the other three, and refuses their advice and takes matters into er own hands when Nate causes trouble for Lewis.
More friction arises, this time over Lewis' birthday party. Charlotte takes control and drives Rikki away from the party. However, things go out of hand and it is up to her to save the day.
Angry over the short friendship with the girls, Charlotte forces Lewis to choose between her and his friends. This results in Cleo to feel isolated and she runs away and gets lost. When he is informed, Lewis must make up his mind: Cleo or Charlotte?
A unusual and rare planetary alingment is set to cause the full moon to remove a mermaids powers if they enter the moon pool! Charlotte shows immunity and tries to reveal the girls secret before Lewis takes her to Mako Island. The girls follow and a battle erupts. But who will win?
