Emma has been busy organsing a surprise birthday party for her dad and when the evening finally arrives it looks like everything is going to plan. But when the clouds part to reveal a full moon something funny comes over Emma. Spellbound with full-moon madness, Emma begins to act very strangely, much to the dismay of the other party guests. Cleo, Rikki and Lewis must do their best to stop their friend from flirting with boys and guzzling down her weight in sardines! But more importantly, can they stop Emma from revealing to a room full of people that she's a mermaid?