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 marriage to his would-be fiancée, 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's 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's dismay, Jessie and Jessiebelle accidentally get switched, leaving James on the run from his crazed fiancée...with Meowth running along with him! And, tired of the nuptial nightmare, our heroes once again turn toward their upcoming challenges.
In preparation for the Daybreak Town Contest, Dawn and Piplup battle against Ash. Ash's Monferno Digs to avoid an oncoming Bubble Beam, but the match stops when it uncovers a buried treasure chest. They all try to break it open, but it is too strong. Team Rocket sees them trying to break it open, and decide to steal the chest to get the treasure inside. They disguise themselves as the Treasure Chest Gang, but they soon reveal themselves as Team Rocket, and Ash has Pikachu send them off with a Thunderbolt. When they land, James remembers that he used to own the treasure chest, and buried it when he was a child, but he can't remember what he hid inside it. The team decides to retrieve the item again, imagining it contains some of James's allowance.
Team Rocket shows up once more, and they try to convince the twerps that the chest belonged to James. Ash and his friends assume it is a lie, but Brock says that if it did belong to James, then he would know how to open it. James tries to open the chest, but he can't remember how. He tries different passwords, and when he tries for the third time, the chests looks like it is opening, but it's really the security system. James tells them to run, but the cannon blasts the trio off again. When they land, James suddenly remembers what was inside the chest, but he won't tell Jessie or Meowth. He becomes determined to stop anyone from finding out what is inside the chest.
Meanwhile, Ash and his friends arrive at the gates of an impressive-looking mansion, when suddenly the gates open and a butler, Sebastian runs towards them. He immediately recognizes the chest, and turns it over to reveal James’s drawing of Growlie on the bottom. The butler gets very emotional at the thought of James returning, and starts to look for him in the bushes and trees. Ash and Brock inform Dawn that James came from a very rich family, and that the huge mansion is probably one of his family's summer estates. Peering through his binoculars, James fears that his butler will call his parents and his fiancée, Jessebelle, whose controlling personality was the reason he ran away from home and joined Team Rocket. James comes up with a plan to retrieve the chest without being recognized by Sebastian.
Inside, Sebastian offers a meal to Ash and his friends as a thank you for finding master James's treasure chest, and while they are tucking in, Sebastian asks them where they found the chest. Before they can answer, the doorbell rings. It is Jessie, disguised as Jessebelle, flanked by Meowth and James as her servants. Jessie says that she has just come to pick up the treasure chest for James, who had returned and was marrying her after all. Sebastian begins to cry with happiness, and Meowth orders Ash to fetch the chest. The plan was working, until Growlie wanders out and recognizes James even through his disguise. Growlie jumps onto James to say hello, and knocks the disguise off. Even though James is scared about being found out, he is pleased to see his old Pokémon again. Sebastian is overjoyed to see James, but he quickly sends out a Spinarak which he tells to use String Shot on James so he can be handed over to his parents and marry Jessebelle. James sends out his Mime Jr. which uses Teeter Dance, which allows James to send Sebastian and Spinarak down a hidden trap door into a dungeon.
James takes the chest from Ash, and when asked by Dawn and Jessie, he tells them what is inside the chest; a love letter he wrote to Jessebelle. He explains that the pair met when they were young during one of his parents’ balls, however the meeting was arranged. James fell in love with her right away, even though he only knew her name. That night he wrote a love letter to Jessebelle, asking her to marry him. He thought how embarrassing it would be if anyone other than her read the note, and just then his parents, who had been listening to him, handed him the treasure chest, which was handmade by his father. They also told him that he would be going to Jessebelle's house the next night, and he should tell her how he feels. The next night, before James got a chance to ask Jessebelle to marry him, she said yes, and that it has already been arranged by their parents. She demanded that he gets a Skitty to replace his Growlithe, but James refused to part with his best friend, and ran out of the house. He told Growlie to use Dig, and buried the chest so no one would find it. James says that if Jessebelle was to find the letter, she would see it as a marriage certificate, and he would have to spend the rest of his life with her.
Suddenly, the security systems go off; it is Jessebelle, and she is making her way to the mansion along with hundreds of guards. The guards surround the building, so James can't escape. James realizes Sebastian must have called her. James says that he has a secret passageway they can escape through. At the same time, Jessebelle enters the mansion, where she is greeted by Sebastian, who jumps up through the floor. He leads her to the secret passageway, where James and the others are all running from her. James tells everyone to hurry up, and Dawn says that she can't see what any of it has to do with them. James reminds them that none of it would have happened if Ash hadn't dug the chest up in the first place, so they should help him escape. They see Jessebelle and Sebastian running close behind, and quickly get to the end of the tunnel, trapping Jessebelle and Sebastian inside. Ash and Brock tell James to run while he has the chance, but James says that he has to dispose of the letter first, and he unsuccessfully tries to guess the password again. This time the chest transforms into a huge and scary looking robot, but really it is only a foot tall. It shoots at Team Rocket, but Growlie uses Flamethrower and stops the laser. James gets Growlie to attack the chest, and once Growlie has used Flamethrower, Dig, then one last Flamethrower, the robot turns back into a chest before transforming into a tiny rocket and flying away.
Inside the tunnel, Jessebelle gets her Vileplume to use Solar Beam, which breaks down the door. Jessebelle tells James that all the arrangements for the wedding have been made, and James says that he gives up, and just wants to say goodbye to his friends before he goes to get married, but James asks Ash to use Thunderbolt. Ash agrees, and Pikachu sends Team Rocket blasting off. James is happy to have escaped, but suddenly realizes that it is not Jessie who has blasted off with them, but rather Jessebelle. Jessie lands on her own, surrounded by Jessebelle's guards, who mistake her for Jessebelle. Later, Ash and his friends make their way to the next city, while James and Meowth are being chased by Jessebelle.