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.

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.

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.

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.

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.

Cleo and Kim are stuck with their cousin Angela who wants to go body boarding and causes a bird brained distraction when Cleo refuses.

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.

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

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

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.

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.

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

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

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.

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.

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.

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 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.

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

Bella tries to get close to Will but when Bella goes to his place she sees him with another girl. Meanwhile Lewis tries to test the water from the moon pool but gets caught by his teacher.

Cleo's family and Sam go on a picnic to the beach, which leads to tension when Cleo refuses to go near the water. Kim abuses the fact that her father is happy in order to get lots of gifts. Will's sister Sophie competes with Bella to get the job Bella wanted at Rikki's Cafe.

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.

Cleo starts to experiment on the moon pool water without Lewis' knowledge, but things start going wrong when Cleo's fish Hector gets trapped in the bubble of floating water. Meanwhile, Lewis has to play golf with Cleo's father.

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.

Bella helps Will with his free-diving training but when she realises he's more interested in her as a mermaid than a girl, she's hurt. Rikki and Cleo have to improvise when the cafe's first corporate function is actually a children‟s party.

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.

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.