The Marine Park is hosting the "Miss Sea Queen Beauty Pageant" and Cleo is planning the make the most spectacular costume the world has ever seen just so she can beat Miriam, who is desperate to win. Cleo is so distracted with putting her costume together that she doesn't realise that her little sister, Kim, has stolen her diary! When Kim reads the diary she's shocked to discover that Cleo and her friends are mermaids, and hatches a plan to expose them for what they really are in front of everyone! Can Cleo and the girls keep their secret safe?