At the beach by Mouseford Academy, one of the Thea Sisters discovers a mysterious boy with blue skin washed up on the shore. He speaks an unfamiliar language, so the Thea Sisters call Thea Stilton to help figure out how to communicate with him. She sets the mouselings on an exciting journey across the world to hunt for clues to the boy's origins. Could he be prince of the underwater world of Atlantis? It's up to the Thea Sisters to get him home!