From the author of the Waterstones Children's Prize shortlisted Once Upon a Unicorn Horn comes a beautiful story of friendship and protecting the oceans. Theodore has a little boat and a big passion: collecting fish. He loves nothing better than discovering a new fish for his collection.

But one day, he finds something he's never seen before: a tiny creature in a beautiful shell. Ignoring the voice that tells him to leave her alone, he takes her home, but in a tank she gets weaker and weaker. Can Theodore learn that the creature belongs in the ocean, before it's too late? This heart-warming story about how mermaids got their tails is the follow-up to the critically acclaimed and best-selling Once Upon A Unicorn Horn and Once Upon a Dragon's Fire.