An all-new hardcover adventure featuring Thea Stilton and the Thea Sisters! At the beach by Mouseford Academy, the Thea Sisters discover a mysterious boy with blue skin washed up on the shore. He speaks an unfamiliar language, so the five mice call Thea Stilton for help. She sets the mouselings on an exciting journey to hunt for clues about the boy's origins. Could he be the prince of the underwater world of Atlantis? It's up to the Thea Sisters to get him home!