When Katie Terosie learns that her family is moving to St. Thomas she is devastated. That is, until she meets her new neighbors Seth and Lauren. While snorkeling, Katie finds a map in a bottle at the bottom of the Caribbean Sea. The kids decide to follow the map and soon find that they can magically breathe underwater. While this adventure excites them, they realize they are in a tricky position. Surrounded by mermaids, mermen, and all kinds of dangers, the only way to get home is to find the King of this magical world and ask for his help. Will he choose to help, or will they be forced to stay like all the other humans - as prisoners?