Immediately after Ash had turned into a Pikachu, he started feeling down that his career of Pokemon training is over, and believing that there is nobody else that knows how he feels about being stuck in his new babyish body when a friendly and playful Shinx named Katie meets him and Pikachu and tells him that being a baby Pokemon in diapers really isn't so bad. Especially for such a cute little Pikachu.
His spirits lifted, Ash starts filling his diapers cheerfully while Katie encourages him to have fun with being a baby Pokemon.
