Ditto's Mysterious Mansion (Japanese: メタモンとものまねむすめMetamon and the Copycat Girl) is the 37th episode of the Pokémon anime. It was first broadcast in Japan on December 9, 1997 and in the United States on October 26, 1998.
Being caught in a sudden rain shower, our heroes are obliged to seek shelter. In the middle of a clearing, they find a small, ghastly cabin. Dragging the reluctant Misty, Ash and Brock enter the cabin, instantly getting the impression that they are not alone. Could this be a haunted cabin? They soon discover that it is the Copycat house in which the trainer of Ditto lives. Intrigued by Ditto, Ash proposes a battle between Ditto and Pikachu.
When it begins to rain, Ash and his friends stumble across an old house in the middle of nowhere. Believing it to be deserted, they go inside. To their surprise, Pikachu plays with what seems to be another Pikachu. However, this Pikachu's face is weird, which leads to Brock thinking it is a new kind of Pikachu. Ash tries to catch it, but a girl dressed as Ash appears and stops him. The girl introduces herself as Duplica and reveals that the Pikachu is actually her Ditto. The mansion is called the "The House of Imite" and serves as a rest stop and performance space for her imitation shows. Upon reading Ditto's entry in the Pokédex and hearing that it cannot learn any other attacks besides Transform, Ash is unconvinced of its usefulness in a battle, so Duplica challenges him to one. Ash chooses Bulbasaur and tries to attack using Razor Leaf, but Duplica's Ditto Transforms and wins using a Vine Whip attack. Later, Duplica explains that she wants to be a Ditto Master and a star. However, she cannot achieve this goal because her Ditto is unable to mirror other Pokémon' faces and always retains the face of a Ditto, much to the disappointment of her audiences.
Then, Team Rocket appears and tries to steal Ditto, in the hopes that they can get it to transform into Dratini and present it to their boss as a Legendary Pokémon. Using Weezing's Smokescreen, they are able to escape with Ditto. Once they are away from the house, Jessie and James try to test Ditto's power by having it turn into a Dratini from a picture in a book that they have. However, Jessie suddenly says that they should have a little bit of fun before handing Ditto over to the boss. She asks Ditto to transform into what a certain boy would look like if he were an adult, and pulls out a photo frame. James and Meowth then try to get Jessie to show the picture to them, but she refuses. Ditto agrees, and transforms into a picture of a man with blue hair, though it has Ditto's face. When it licks Jessie's face, James and Meowth laugh hysterically, but Jessie responds by furiously slapping them unconscious. She then commands Ditto to become a Dratini, but instead, it transforms into a book, which aggravates Jessie even further.
Meanwhile, back at the mansion, Ash talks about how a Trainer decides how a Ditto would transform. Pidgeotto and Zubat, who were on guard, return with news that they saw Ditto. Ash and his friends then decide to go and save Ditto. Elsewhere, Team Rocket, frustrated at Ditto's inability to fully transform into whatever they wish, threaten it into transforming correctly, pleasing them. Just then, Ash, Brock, Misty, and Duplica, all dressed in Team Rocket uniforms, arrive and perform their own variation of Team Rocket's motto, much to the anger of the real trio. The group then demands Ditto back, but are surprised to see two identical Meowth. However, Duplica is happy that Team Rocket was able to get her Ditto to transform correctly. Team Rocket tries to trick them by giving them the real Meowth and escaping in their balloon with Ditto. However, this fails when Duplica says she can tell that Meowth isn't her Ditto, even though it looks identical to the other Meowth. She throws Meowth at the balloon, causing Team Rocket to drop Ditto. Duplica tells it to transform into a cannon and fire Pikachu at the balloon. After being hit by Pikachu's Thunder Shock, the balloon bursts and Team Rocket is blasted off again. Now that Ditto can transform perfectly, Duplica reopens her theater for performances. She thanks Ash and his friends, and they leave, with Ash hoping to catch his own Ditto someday.
Meanwhile, Jessie and James attempt to carry out their original plan without Ditto and put Meowth into a Dratini suit to this end, though he is unimpressed and scratches them.
Duplica claims that Ditto is the only Pokémon that can use Transform, but Mew shares this ability. However, since Mew was at this point a secret Pokémon, the writers probably made Duplica not know about Mew's existence.
When Duplica imitates Ash, she is even wearing a hat just like his; this shows her knack for getting even rare outfits for her mimicry, as there were only 100 given away.
When Duplica imitates Officer Jenny, she says "My name's Jenny. You're under arrest for Pokémon poaching." This is a shortened version of what the Officer Jenny stationed in the Safari Zone said to the gang in The Kangaskhan Kid and is most likely the same audio, due to the exact same inflections.
As James furiously points out, when Ash and his friends imitate Team Rocket, Duplica incorrectly says "To ignite all peoples within our nation."
It is revealed that Ditto can also transform into inanimate objects like books, photo-frames, etc.
It is also revealed that Ditto cannot transform into a Pokémon from a picture, so there must be a real one around for it to imitate.
Also, since it gains all the abilities of the Pokémon it transforms into, if that Pokémon has learned to speak human language then Ditto can temporarily gain that ability for itself.