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.

Emma gets into trouble babysitting her little brother Elliot. She goes to Lewis for help when Elliot develops a crush on Rikki.

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.

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.

Emma invites Miss Chathams to stay at her place and Zane overhears Miss Chathams concerning about 'her treasure'.