As Ash prepares for his final Sinnoh region Gym Battle with a little training, his Monferno accidentally discovers a treasure chest buried in the forest! While struggling to open it, our heroes are approached by Team Rocket-in disguise, as usual. James remembers that he was the one who buried the treasure chest long ago! James and the rest of Team Rocket attempt to retrieve the chest, but to no avail.
Meanwhile, James recalls that the chest contains a letter proposing to his would-be fiancee, Jessiebelle. James knows if that letter ever falls into her hands, the results would be disastrous! And our heroes, who still have possession of the chest, are wandering by one of James' vacation mansions when Sebastian, the butler, spies them. Excited at the thought that "Young Master James" must be close by, Sebastian invites our heroes in, while Jessie, disguised as Jessiebelle (along with a disguised James and Meowth), shows up to get the treasure chest back.
But when Sebastian discovers James is really there, he takes it upon himself to alert the real Jessiebelle, who arrives on the scene-all set to marry James! James sadly gives in and says goodbye to Team Rocket and our heroes. But, secretly, he asks Ash to have Pikachu use Thunderbolt and blast them off, away from Jessiebelle! To James' dismay, Jessie and Jessiebelle accidentally get switched, leaving James on the run from his crazed fiancee... with Meowth running along with him! And, tired of the nuptial nightmare, our heroes once again turn toward their upcoming challenges.