Pokemon - episode 37 - 08 Dec, 1997

During a storm, Ash and his friends arrive at a mysterious mansion. While there, Ash meets Duplica (イミテ, Imite), a girl who is an expert at impersonation, and she happens to own Ditto, a Pokémon that can transform into any Pokémon. However, Ditto cannot seem to figure out how to change its face, a fact that stopped Duplica's variety show. As they all try to help Ditto perfect its transformation, Team Rocket tries to steal the Ditto so it can transform into a Dratini which they can give to the boss. (Source: Wikipedia)