After the Team Plasma escapades, Ash decides to return to Kanto with Iris and Cilan wanting to go there to continue their quests to be Dragon Master and Pokémon Connoisseur respectively. Following Juniper's suggestion, Ash & Co. decide to take a ferry which goes through the Decolora Islands. However, when they get on the ferry, they soon learn that everything is not as it seems when simple doors refuse to open. It turns out Team Rocket are in command of the ship...

Traveling to the next island in the Decolora Islands, Ash & Co. participate in a small tournament on the ship. After the tournament, a gem collector intends to show off her special gem collection, and her Purrloin seems jealous of one of the jewels. Another gem collector is onboard and continually asks the collector to sell him her special Liepard's Eye gem, but she refuses. However, the gem soon goes missing and accusations fly everywhere. Cilan decides it is down to him to find out who stole the gem...