Fish-tastic! An Invitation to the Ryugu Palace!?

Synopsis

Fleeing a huge swarm of jellyfish while swimming in the sea, Van, Kappei, and Jane wash up on a beach and find a turtle stuck on its back, unable to move. When they help the turtle, it says that as thanks it will take the three of them to the "Ryugu Palace." In the dazzling Ryugu Palace, the three are welcomed by fishfolk and mermaids and have a wonderful time. Meanwhile, Kappei, fearing an ending like "Urashima Taro," sits alone stewing in his own suspicions…?