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 PokeBlocks 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?