Thora isn't your average ten-year-old. She's half-mermaid and half-human. Now that Thora's living in London with her mermaid mother and human caretaker, Mr. Walters - not to mention a pet peacock and a rare green sparkling sea-unicorn - it doesn't take long for things on land to go awry. Especially the disturbing disappearance of their houseboat, the Loki, and her sea-unicorn, Shirley!

When Mr. Walters's friend at Snug Harbor invites her family for a respite, Thora discovers more snags - like a girl named Louella who doesn't believe in mermaids. While Louella tries to stop Snug Harbor from being converted into a croquet park, Thora thinks she can help. But will she be able to get to the bottom of her own troubles and find her beloved Shirley?

The second book about Thora is another enchanting tale of a good-humored, extraordinary young girl with a knack for unraveling fishy mysteries.