Emily Windsnap lives on a boat, but her mother has always been oddly anxious to keep her out of the water. It is only when Emily has her first school swimming lesson that she discovers why: as soon as she gets into the water, she grows a tail! Soon Emily discovers a glorious underwater world of fishes, coral, shipwrecks and mermaids, and, best of all, she finds a best friend! With mermaid Shona Silkfin by her side, Emily uncovers a surprising family secret and embarks on a quest to reunite her mum and dad.