The marine park is hosting the Miss Sea Queen Beauty Pageant and Cleo plans to make the most spectacular costume the world has ever seen so that she can beat Miriam, who is desperate to win. But Cleo is so distracted putting her costume together, that she doesn't notice her little sister, Kim, stealing her diary! Kim is shocked to read that Cleo and her friends are mermaids and hatches a plan to expose them for what they really are in front of everyone.