Meet Isadora Moon's mermaid friend, Emerald! Emerald is learning how to be a mermaid princess, but there are just so many rules! She loves having fun with her friends and exploring her underwater world with her pet octopus, Inkibelle, she doesn't want to be stuck in the palace with everyone looking at her and checking that she's doing things the proper, royal way.When Emerald and her stepsister, Delphina, sneak out of the palace in search of sea sprites they know it isn't strictly 'perfect mermaid princess' behaviour, but they can't resist! Far from Scallop City they discover a glimmering coral reef. And nestled in the reef are three little shimmering sea sprites . . . Emerald and Delphina are about to learn that being a mermaid princess is about more than just following the rules - it's also about following your heart.