A missing magical pony, lost treasure, a mysterious sea monster and a vanished princess. When there's a mystery to be solved, the young mermaid detectives are always right on the case! The mermaid friends are excited about the annual carnival that is to be held in Mermaid Bay, especially as the team who put on the best display will win a beautiful pearl necklace. Rosa comes up with the idea of using magic to create a water-pony and plans to perform a series of amazing tricks on its back. But just before the carnival is due to start, the magical pony is stolen. Who is trying to sabotage the friends' performance and why?