Cleo's secret diary falls into the hands of her younger sister, Kim. Kim grows suspicious when she reads in the diary that Cleo and her friends are mermaids, and shares her ideas with Emma's brother, Elliot. At first, Elliot doesn't believe her; but when Emma accidentally freezes his shower, Elliot becomes convinced that his sister is part of an evil mermaid cult. To put the kids off the scent, the girls decide to make them think that the diary is about someone else, and choose Miriam as their decoy, using their powers to make it look as though Miriam has powers of her own. Meanwhile, an oblivious Miriam enters the Miss Sea Queen pageant, and taunts Cleo, who decides to enter the pageant herself - unaware that Kim and Elliot are planning to shower the stage with water to expose Miriam and Cleo as mermaids. The plan ultimately fails and Miriam is humiliated. Meanwhile, Lewis goes to Mako Island to investigate the moon pool in order to find out how the girls became mermaids.