Shelly Siren loves playing on Trident Academy's Shell Wars team. But is she ready to compete in the Shell Wars Championships in the fintastically famous city of Atlantis? She's not sure, but luckily Echo, Kiki, and Pearl will be swimming along for the ride.

As part of their trip, the mergirls get a tour of Atlantis, and learn all about how the great human city ended up at the bottom of the ocean. But when they swim through Poseidon's...