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. The morning after a big storm, Emerald discovers a shimmery egg in Dad's back garden.

A baby sea dragon hatches from the egg! It is sooo cute! When the cheeky little sea dragon hitches a lift with Emerald back to Mum's, everyone at the royal palace is enchanted. Emerald knows deep down that they need to find its real family and so Emerald, Delphina, and Mum set out on an exciting ocean adventure to do just that.