Police Officer Gardie : Season 1 Episode 54

When Satoshi and his friends see a group of Gardie training against a Policeman posing as a robber, Satoshi decides for him and Pikachu to do the same training to become the best they can be. However, when the Rocket-Dan comes to steal them, Officer Junsa, Satoshi and the gang's voice gets changed, causing the Gardie to think that the Rocket-Dan are the Officers as they use their ears to follow command.