On the way to the Grand Festival, our heroes are swarmed by a group of Aipom who swipe Ash's hat and May's ribbon case! That's not all—the Aipom also cleaned out an orchard belonging to a farmer, Taylor. He says the local Pokémon have started acting strangely, and it might have something to do with a strange monster recently seen in the mountains. Everyone sets out to investigate and get May's ribbons back.
Up in the mountains, the Aipom have brought their loot as a tribute to the monster, a Mountain King who's represented by none other than Team Rocket. Ash and his friends show up and challenge them, but then the Mountain King appears: a giant Slaking! Team Rocket orders the Pokémon to chase off our heroes in the name of the Mountain King, and everyone splits up to get away. In the commotion, Ash saves the Aipom who took his hat, then runs off and to find his friends.
Ash and his friends regroup and confront Team Rocket, but their giant Slaking grabs May! Ash's Pokémon get her to safety and Combusken's flames burn the Slaking to reveal that it's a giant robot. Aipom shows up with the other Pokémon and help wreck the robot now that Team Rocket's tricks have been revealed. Then it's just a matter of convincing Team Rocket to return May's ribbons and leave the scene—with encouragement from Combusken and Pikachu. Taylor gets his Berries back and Ash gets his hat... as well as a new fan!
A magnificent mountain lights the sky as Ash and his friends make headway towards the Kanto Grand Festival. After Ash's defeat at the Battle Pyramid while under the influence of the spirit of the King of Pokélantis, our heroes look forward to May's participation in the Festival and Ash's future attempt to defeat Brandon. May opens her Ribbon case, displaying her five Ribbons glistening in the sunlight.
Down the road behind them, a man calls out to a group of Aipom running off with his watering can and Berries. Some of the Aipom jump on the group's heads, including one who snatches May's Ribbon case with its tail and another that discards the watering can and takes Ash's hat. The Aipom run off as the old man catches up with the group and checks if they are okay.
They head back to a farmhouse, where the man introduces himself as Taylor and treats Ash and his friends to hot beverages. Taylor says that he grows Berries and makes them into Pokéblock to sell. Ash questions the man about the Aipom, who Taylor says are part of a group of Pokémon that live in the mountains and would only eat from his trees when they were hungry. Lately, however, they have been taking all of his produce. He then tells a story about a traveler who became lost on the mountain roads on a dark and foggy night. As he moved closer to the mountain and a wooden wall, lightning erupts in the sky, showing him a silhouette of what looks like a giant monster. The man runs away, eventually finding Taylor and telling him about the creature. Taylor does not know if the monster is involved with the Pokémon taking his Berries, but they all agree to head up to the mountain, uncover the mysteries, and regain their possessions.
Meanwhile, Meowth and Wobbuffet are performing a small ritual as the five Aipom present their stolen goods. Meowth wears a pair of geta, making a clacking sound while Wobbuffet has on a red and white hakama. When the large, wooden doors open, the monster's silhouette appears and roars, making the Aipom scatter in fear. Meowth and Wobbuffet are pulled inside and the doors shut. Jessie and James finally reveal themselves examining the goods obtained from the Aipom. James finds May's Ribbon case, which Jessie grabs and excitedly proclaims is her ticket to the Grand Festival. Meowth and James are shocked by her determination, while Wobbuffet simply calls out affirmatively.
Our heroes are headed towards the area of the disturbance as a group of Mankey scurry across their path with more Berries. The other mountain Pokémon, some Vigoroth and Slaking, present Berries to the disguised Meowth and Wobbuffet as well. Ash and company observe from a bush and see the Aipom wearing Ash's hat. The doors open again, partially revealing the monster, frightening the mountain Pokémon away. Before the disguised Team Rocket Pokémon escape, May runs out to get inside. Everyone else follows and they barely make it. May questions Meowth about her Ribbon case, but Jessie shows it to her arrogantly. She continues to taunt May, saying that she'll use her Ribbons to win the Grand Festival.
The monster comes forth and reveals itself to be an enormous Slaking. The sight frightens the group as the creature causes tremors by stomping. The wooden doors open again as the mountain Pokémon return to the scene. The giant Slaking roars, which Meowth interprets as a command to get rid of the "Twerps." The Pokémon obey, chase them away, and eventually encircle them on the road. Ash has Pikachu warn them with a Thunderbolt aimed at the ground. Taylor suggests they split up, so he goes down the road with May and Max while Ash, Brock, and Pikachu head in the opposite direction. The Aipom pursue the latter group while the rest go after Taylor, May, and Max. As they flee, Brock releases some Pokémon food to occupy the Aipom. The Aipom with Ash's hat keep chasing them and jumps on Ash's head, knocking him down a hill. As they both tumble and slide, Ash grabs a small tree growing from the side and saves Aipom from the fall. After a few moments, the branch begins to break and they fall again, but are cushioned by a thick bush. Noticing that the Aipom has lost his hat, Ash goes after it, but Aipom beats him to it and places it atop her head once more. Meanwhile, the other group has managed to lose their pursuers by hiding in a bush.
After cleaning himself in a nearby river, Ash tells the now-friendly Aipom that she cannot trust Team Rocket. Brock and Pikachu come running from the distance and join him in returning to the wooden wall. Heeding Ash's advice and still wearing his hat, Aipom rejoins her comrades as they eat Brock's food. Aipom begins to talk as the other mountain Pokémon show up. The gigantic Slaking finishes devouring the food and lifts Team Rocket on top of its shoulder to begin their trek toward the Indigo Plateau. Taylor, May, and Max climb over the wall using ropes and confront Team Rocket, whereupon Jessie taunts May again with her Ribbons. Slaking begins moving forward defiantly, so May calls upon Combusken to stop it with Fire Spin. Slaking merely blows a gust of wind to dissipate the flames, tears down the wooden wall, and continues down the path. The other group arrives and Pikachu blasts a Thunderbolt at Slaking's feet, prompting Team Rocket to perform their motto. Slaking extends its hand, grabs May, and begins climbing a nearby tower of rock. Ash releases Swellow to save May. Slaking reaches the top and swats at Swellow, but managing to drop May in the process. Thinking quickly, Ash calls upon Sceptile to catch May just in time. Swellow then knocks Slaking and Team Rocket off of the tower with Aerial Ace.
May furiously gets her revenge by directing Combusken to attack with Fire Spin. Brock releases his Marshtomp to douse Slaking off with Water Gun. The giant Slaking is revealed to be a giant robot, now with Team Rocket inside. The Slaking mecha begins banging its chest as the Mankey, Vigoroth, Slaking, and Aipom return and attack with thrown rocks, tossed large stones, banging fists, and Scratches, respectively. After a few moments, the Pokémon flee from the mecha as its joints explode, releasing the stolen food and ejecting its occupants. Jessie then shows May her Ribbon case once more in arrogance and calls upon Seviper to attack with Bite. Combusken hits first with Quick Attack, however. Seviper is told to Bite again, but Combusken dodges and strikes with Sky Uppercut. Seviper plows into Jessie, who lets go of the Ribbon case to be caught by May, who is relieved to have it back. With Meowth's urging, James releases Cacnea and gets another spiny hug before directing it to use Needle Arm. Pikachu ends the confrontation with a powerful Volt Tackle, sending Cacnea off with such force that it takes Team Rocket along with it into the sky.
The flames of courage burn brightly once more as our heroes head for the Grand Festival after the Pokémon and Taylor wish them a grand parting. Ash continues down the path still without his hat, but notices it atop a rock structure along with a pink flower and an apple. He remarks on Aipom being a cool Pokémon after all and sets off again on the road. In a nearby tree, Aipom spies on Ash and smiles.