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 even makes a mermaid best friend, Shona Silkfin! With Shona and her other friends by her side, Emily embarks on a series of exciting and magical adventures, tackling everything from an angry sea god to a monstrous kraken.