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. Emerald's class are spending the day sea combing-picking up human rubbish that ends up on the ocean floor. When Emerald finds a precious teddy lying in a clump of seaweed, she's tempted to keep it for herself, but the tag says it belongs to a human called Jack.

Emerald asks her land-loving friend, Isadora Moon, and Isadora's cousin, Mirabelle, to help her return it and together they embark on an exciting adventure across the land and sea!