As if being a teen weren't hard enough, the three gals in this Australian fantasy series must cope with the added burden of growing a giant fin and transforming into mermaids whenever they come into contact with water -- a fact they try to hide.
Cleo and Emma meet new girl, Rikki, and find themselves stuck on Mako Island where they are transformed into mermaids. As well as growing tails, the girls find they have powers over water.
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.
Rikki, Emma and Cleo discover that sea turtles are being captured by fishing nets. When Rikki and Emma attempt to rescue the turtles by cutting the nets, it causes the fisherman to think that there is a rogue shark roaming their ocean.
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.
Rikki gains an admirer in Emma's younger brother Elliot when she saves him from drowning. She lets him down hard causing him to run away and hide in the fernery. The girls find him just as the watering system is about to start…
The effects of the full moon turns Emma into a carefree, wild mermaid. At her father's birthday party she's the opposite to her usual, organised self. Can the other girls stop her before all is revealed?
A charismatic newcomer, Dr. Denman, tempts Lewis with the promise of scientific fame and fortune. Will he abandon his friends and risk their secret, all in the name of science?
Rikki uses her abilities as a mermaid to earn some quick cash, but when she accidentally becomes involved in a fish smuggling operation, she sets out to get her revenge.
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.