On the way to Indigo Plateau for May's Grand Festival, a group of Aipom come by and steal Ash's hat and May's ribbons! An old man comes by and tells them that local Aipom, Mankey and Vigoroth have been stealing his berries and taking them up to the mountains. He also tells them that there is a rumor about a giant monster living in the mountains. They follow the Aipom into the mountains to a giant wooden face. When the fence opens up, they run in and find out that it is really a Giant Slaking working with Team Rocket! They plan to use all the berries to make and sell Pokéblocks at the Grand Festival, and Jessie will use May's ribbons to compete! After being chased out of the Slaking's territory, Ash and Brock are separated from everybody else along with the Aipom that stole Ash's hat. Can they reunite, get May's ribbons back, solve the mystery of the Giant Slaking, and get Ash's hat back?
Ash already plans to train at Indigo Plateau for his battle with Brandon. May also plans to win the Grand Festival. Max reminds her there will be a tough competition, but May has 5 ribbons, a proof she is able to win. Suddenly, they see a man chasing a group of Aipom. The Aipom jump on Ash's head, and one of them steals his hat and May's ribbon case. The Aipom disappear, making May nervous. The man takes them into his lodge, introducing himself as Taylor, who wanted to make Pokéblocks from the berries Aipom stole. The heroes introduce themselves as well. Taylor tells them Aipom were taking the berries when they were hungry, but later they were stealing for no purpose. And not only Aipom - there are Vigoroth, Mankey and Slaking.
Taylor doubts the Pokémon don't have food, as a traveler got lost and saw a wooden gate and saw a monster and ran off. Taylor decides to accompany the heroes to see the monster and retrieve the stuff back. Team Meowth and Wobbuffet (dressed) tell the Aipom to give the stuff for the king. They give to them the berries and the gate opens. The monster scares them and Meowth and Wobbuffet are pulled inside. Jessie and James come to them and see the new batch of berries so they can sell them as Pokéblocks at Grand Festival. Jessie sees a badge case with all 5 ribbons and decides to use them at the Festival.
The heroes follow Taylor and see a group of Mankey stealing berries. So, they decide to follow them. They spot the Aipom (and Ash's hat), Vigoroth, Slaking and Mankey giving the berries. Wobbuffet and Meowth chant, so the gate opens and the Pokémon run away, while Meowth and Wobbuffet are pulled in. May is impatient and goes in, so others follow her as well, just in time for the gate to close. They see Team Rocket and May demands her badge case back, though Jessie does not give it to her. Ash wants to know what is the plan, though Meowth tells him they are serving the Mountain King, who is a giant Slaking. Slaking stomps, shaking the ground. The Pokémon who gave berries see it and are taken back. Meowth translates the King wants to dispose of the twerps, so the Pokémon attack them.
The heroes run away from the Pokémon, but are stopped when they form blockades. Pikachu uses Thunderbolt to frighten the Pokémon, who get away. Ash and Brock go in one direction, while Taylor, May and Max the other and the Pokémon follow them. Ash sees the Aipom following them, so Brock drops a portion of food to stop them. However, the Aipom with Ash's hat follows them. Ash and Brock come to a cliff and the Aipom jumps to Ash. Ash struggles to get his hat back, but the Aipom refuses, resulting in both of them falling down. Ash hangs on a branch and pulls Aipom, but the branch is destroyed and both Ash and Aipom fall on a bush. Ash goes to get his hat, but the Aipom gets it first, but this time, it likes Ash.
Meanwhile, May, Max and Taylor hide from the bunch of Vigoroth, Slaking and Mankey. They lost them, but May needs her ribbons back. Ash washes his face and sees Aipom likes his hat. He tells it shouldn't trust Team Rocket. Brock and Pikachu arrive, so Ash tells it they need to gather the berries, leaving it with the hat, though Aipom climbs the cliff up. Ash tells Brock they need to convince the Slaking Team Rocket use it for their own needs. The Aipom comes to other Pokémon and tells what Ash said. Meanwhile, the giant Slaking eats the berries and allows Team Rocket to be on its shoulder. However, May, Max and Taylor arrive. They demand the ribbon case and the berries, but Team Rocket will not give it to them and tell they stand in Slaking's way. Slaking walks to them, so May sends Combusken, who uses Fire Spin to stop it, but Slaking blows the attack away.
Slaking destroys the fence, but Pikachu uses Thunderbolt to stop it. Slaking ate all the berries, so they need to stop it from harassing everyone. Team Rocket demands Pikachu, but naturally, Ash will not hand him over. Slaking goes to reach and takes May. Slaking climbs a nearby boulder, so Ash sends Swellow to stop it. Slaking waves its hand to stop Swellow and drops May, but luckily Ash sent Sceptile to catch her. Swellow uses Aerial Ace, knocking Slaking to the ground. Combusken uses Fire Spin, torching Team Rocket and Slaking. Brock sends Marshtomp, who uses Water Gun on Slaking. The Slaking is actually a robot made by Team Rocket, so the berries were not eaten. Vigoroth, Slaking, Aipom and Mankey came.
The Pokémon throw stones on the robot and scratch it, shaking the robot. Suddenly, the malfunction causes the robot to explode and Team Rocket fall to the ground. The heroes demand the stuff back, but Jessie does not allow it and sends Seviper. Seviper goes to bite, but is stopped by Combusken's Quick Attack. Seviper goes to bite once more, but Combusken evades and uses Sky Uppercut. Seviper is thrown to Jessie and she drops the ribbon case, which May catches. James sends Cacnea to fight. Cacnea uses Needle Arm, but is thrown to Team Rocket (who blast off) with Piakchu's Volt Tackle. Everyone cheers for the victory today. Later, Taylor retrieved all his berries and the heroes need to go. May is excited to get to Indigo Plateau, though Ash feels awkward without his hat. Suddenly, they see his hat lying on a rock. They also see a flower and an apple, leading to belief Aipom gave Ash the hat back. The heroes continue their journey, while the Aipom smiles and follows them.
- Brandon is added to the opening animation.
- During Team Rocket's initial appearance, Meowth makes a reference to King of the Hill, a children's game.
- This episode's name (as well as the event of May being taken by the Slaking robot) is a reference to King Kong movie.
- The hill that Ash and Aipom fall down is initially shown to be a vertical cliff, but becomes a 45-degree decline when they fall.
- The Slaking are shown to be only one foot taller than the Vigoroth initially in the episode, but at the end they are shown to be two feet taller.
- In the English dub, as May is being rescued from Slaking, an arranged version of "Giovanni's Theme" from Pokémon Puzzle League is used.