From the author of the Waterstones Children's Prize-shortlisted Once Upon a Unicorn Horn comes a beautiful story of friendship and protecting our 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 heartwarming 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.