The most popular girl in school is having the party of the year and Cleo is desperate to go. But when Cleo and her friends Rikki and Emma discover the party is actually a pool party, they realise that this may be a problem. Cleo and her friends are part time mermaids and on splash of water will make the girls grow tails. However, Cleo is determined to find a way to carry on with life as normal and go to the party, keeping their mermaid secret safe ... for now!