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 Emma and Rikki discover that the party is actually a pool party, they realise that things could get a little tricky! Something strange and mysterious happens when the girls come into contact with water - they each grow a mermaid tail! So, a pool party is perhaps not such a good idea? However, Cleo is determined to find a way to go to the party...can the girls keep their mermaid secret safe?