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

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.

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.

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.

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

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.

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.

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.

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.

Cleo discovers a comet is causing fish to flee the waters around Mako Island. The girls find a way to restart the message in the Moon Pool and discover the comet will collide with earth. Sophie meets Ryan and with Zane's help they join forces to remove the crystals from the Moon Pool.

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.

Will pushes himself too far when Zane challenges him to prove his skills as a free- diver. The girls discover a unique crystal in the rock from the Moon Pool.

Will competes in the Free-Diving Championships but afterward chooses to break from the sport and shrug off Sophie's influence on his life. Rikki sees Sophie kissing Zane at the event and breaks off their relationship.

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.

Bella's eager to ask Will to the end of year party until she sees him asking Rikki. A big misunderstanding occurs which prompts Will to express his true feelings for Bella. Sophie tries to get Zane to change the name of the cafe.

Three very different teenage girls, Emma, Cleo and Rikki, facing everyday teenage problems. With a twist: they're mermaids; after discovering a magic place where they are given incredible powers over water that change their lives forever.

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.

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.

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.

During a full moon, the girls stay at Cleo's and Bella is attacked by the water creature. At the same time, Will decides to return to the moon pool after making the connection between his previous encounter with the moon pool waterfall and the timing of the full moon.