The strange old woman makes a reappearance at the marine park, and gives Cleo a mysterious warning not to look at the full moon. The other girls, however, are convinced the old woman is crazy, and ignore the warning, going ahead with Emma's planned surprise party for her father. At the party, however, Emma sees the reflection of the full moon in a bowl of punch, and then starts acting very strange, losing her inhibitions and being rude to the guests. Cleo, Rikki and Lewis try to keep her under control, but Emma ends up diving into the water, and turning into a mermaid. Worse still, when the others dry her off, her tail won't go away. How can they hide a mermaid in the middle of a party?