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?

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?

Dr. Denman sets up a trap for the girls at Mako Island and Zane is forced to decide who he should be helping.

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.

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!

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?

Zane's quest to find the 'sea monster' has led him to Mako Island. Rikki loses control of her powers as a result of the full moon and Zane finds her.

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?

Kim attempts to derail Don and Samantha's wedding. Lewis receives an opportunity to study in America. Cleo realises she's got to let Lewis live his own life and let him go overseas.

Emma and Cleo are concerned about Rikki going out with Zane. They force Rikki to decide to either be with Zane or with them.

It's time for Emma's family's annual vacation trip, but she can't go as the risks are too high. But all her excuses don't go very well with her family. Meanwhile, with the return of Dr. Denman, Zane is closer than ever to cracking the girl's secret.

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.

Rikki receives counterfeit money at the cafe and is held captive by criminals on a boat. Bella and Cleo are forced to reveal the trio's secret to Will so they can rescue Rikki together.

Sophie and Ryan damage the Moon Pool in their efforts to obtain the rare crystals. The girls improvise and use their own magic to prevent the impact of the comet and a planet wide catastrophe. Rikki, Cleo and Bella graduate with a surprise.

Emma decides to change her look by colouring her hair red. Though nothing is that simple when you're half-mermaid. Zane asks Rikki out on a date.

Rikki finds a locket that looks the same as Miss Chatham's in a Jewellery shop and she is determined to get it.

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...

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"

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.

The girls have to fake sick as a result of a thunderstorm, so their secret won't be discovered at school. But their 'sickness' attracts the attention of doctors.

Lewis enters the annual fishing derby. When Cleo sees him struggling, she decides to help him out with disastorous consequnces. Meanwhile, Rikki and Zane are trapped on a hotel balcony.

With the school dance coming up, Emma and Cleo hope Byron and Lewis will ask them out. Meanwhile, Miss Chatham and Lewis are arguing about which is more important, magic and science.

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.

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.

After Bella gives Will a shell-like fossil she found while swimming, Will becomes suspicious as the fossil is of a rare kind. Rikki realises how close Will is to finding out their mermaid secret and warns Bella to stay away from Will.