Annoying Miss Popularity, Miriam, is having a pool party and the girls want to go. But being around water is risky, especially when Zane tosses Cleo into the pool. Lewis is the only one around to help so he finds out their secret.

Emma decides that she is not going to have her annual sleepover party due to what has happened to them. But Rikki and Cleo try to convince her otherwise. Meanwhile, Lewis conducts experiments on the girls.

Cleo's sister Kim finds her diary and thinks that Cleo and her friends are 'evil mermaids' and she plans to tell everyone. Meanwhile, Lewis goes to Mako Island to find out more about what happened to the girls.

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.

Byron asks Emma for help with training the school's swimming team and Emma has some trouble deciding whether or not to help out in her current situation. Meanwhile, Lewis continues to experiment on the girls, which annoys Rikki.

The full moon gives Cleo a power to put guys in a 'trance' and she suddenly gets a rush of admirers.

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.

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.

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.

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.