Pokémon Season 1 Episode 26

When Ash reaches the Celadon Gym, he is kicked out because he is a boy. He soon discovers that it is only for girls, so with the help of Team Rocket, Ash sneaks into the gym disguised as a girl. In the gym, his disguise is ruined by Misty, Brock, and Pikachu, but the Gym Leader, Erika grants him a battle. During the battle, Team Rocket interrupts and sets the gym on fire. After rescuing Erika’s Gloom, Ash is presented with the Rainbow Badge.