Ditto's Mysterious Mansion is the 37th episode of the Indigo League season.
Ash, Misty, and Brock are walking along a path when it starts to rain. They take shelter from the rain inside a strange mansion. When they enter it, Pikachu starts playing with another Pikachu. Misty says the Pikachu looks cute and she picks it up, but she sees its face looks very weird, and she drops it, freaking out. Brock thinks the Pikachu might be a new kind of Pikachu. Ash tries to catch it, but a girl that is dressed exactly like Ash stops him. The girl says her name is Duplica and that the strange Pikachu is her Ditto. Ash reads Ditto's entry in the Pokédex and it says the only move Ditto can use is Transform. Ash thinks it's not a very good Pokémon since it can only transform and nothing else, so Duplica challenges Ash to a battle. Ash chooses Bulbasaur and Bulbasaur tries to attack with Razor Leaf, but Ditto transforms into Bulbasaur and uses Vine Whip, defeating Bulbasaur. Ash is surprised that he lost.
Duplica takes Ash and his friends to a room filled with costumes. Duplica goes through a rack and comes out dressed as Nurse Joy. She then goes through another rack and comes out dressed as Officer Jenny. Duplica explains that with all of these costumes, she can imitate anyone perfectly. Brock says that she doesn't have the adult charm of the people she's imitating. Duplica goes through a rack, comes out dressed as Misty, and imitates her perfectly (except the eyes: Misty has blue-green eyes, Duplica has brown eyes).
Ash is still upset that he lost to Duplica and Ditto. Ash wonders how he could have lost to a "fake". Brock explains that when a Ditto transforms into another Pokémon, it copies that Pokémon's moves. Misty says that with Ditto, Duplica could be a Pokémon Master. Duplica says she doesn't want to be a Pokémon Master, she just wants to be a Ditto trainer and a star.
A flashback happens and it shows Duplica and Ditto doing a performance for an audience. Duplica dresses up and imitates some Pokémon. Duplica explains to Ash and his friends that the reason she started doing shows was because she loved to imitate Pokémon. Back in the flashback, Duplica asks for a volunteer Pokémon to come up from the audience. A Machoke comes up and Ditto transforms into it, but its face stays the same. A Voltorb comes up next and when Ditto transforms into it, it still has the same face. People from the audience start to boo and yell that Ditto is a dumb Pokémon and can't transform right. Ditto, transformed as Voltorb, gets mad and uses Selfdestruct.
Duplica says she stopped doing shows because people didn't want to see Ditto's messed up transformations, they want it done right. Suddenly, the lights in the mansion go out and a spotlight turns on on the stage. Team Rocket appears and the members, Jessie, James, and Meowth, say their motto. Ash claps, says the show is over, and tells them to go home. Jessie explains that there here to steal Ditto, Meowth grabs Ditto and Weezing uses Smog. The room fills with smoke and when the smoke is gone, Team Rocket has disappeared.
The scene cuts to Team Rocket who are in a small house. Team Rocket is celebrating because they have finally stolen a Pokémon. James pulls out a book on Pokémon and tells Ditto to transform into a Dratini, explaining that it'd make a handsome gift for the boss. Jessie says that before they send to the boss, they should have some fun with its transformation powers. Jessie holds a picture to Ditto's face and asks Ditto to transform into what the person in the picture would look like as an adult. James asks Jessie whose picture it is, but Jessie refuses. Meowth looks at the picture and notices its a picture of a boy. Jessie says it's a picture of her first boyfriend. Ditto transforms into the picture but it still has its own face. James and Meowth roll on the floor and laugh, saying her first boyfriend has an interesting face. Jessie beats them both up. When Jessie angrily yells at Ditto, the transforming Pokémon licks her face, and James and Meowth laugh again, only to receive another beating. She holds up the Pokémon book with the picture of the Dratini on it and asks Ditto again to transform into Dratini. Ditto transforms into the book, but still has its own face. Team Rocket falls down, anime style. Jessie angrily screeches at Ditto, saying it'd better get it right next time or she with turn Ditto into jelly. Ditto starts whimpering nervously.
Back at the mansion, Ash and his friends are sitting in a circle around Duplica, who is upset. Misty tries to make Duplica feel better and tells her that Pidgeotto and Zubat are out looking for Ditto. Just then, Pidgeotto and Zubat return and they have found out where Ditto is. Duplica says she has a plan to get Ditto back.
Back at the small house, Meowth suggests that Ditto should try to transform into something easier and tells Ditto to transform into himself. Ditto transforms into Meowth, but still has the same face. Meowth says to Ditto that he had better get it right or he will regret it. When Meowth threatens to claw Ditto's stupid face off, Ditto rubs its face and it fixes its face. Ditto has finally learned to transform right! The lights go out and Ash, Misty, Brock, and Duplica are dressed in Team Rocket uniforms and they imitate the Team Rocket motto.
Team Rocket is mad at them because Duplica got the words wrong (in the motto, when Duplica said her first line, she got "unite" wrong and said "ignite"). Ash and Misty now understand why Team Rocket always says their motto, because its fun. Ash, his friends, and Duplica asks for Ditto back, but they can't tell Ditto and Meowth apart. Duplica starts crying and thanks Team Rocket for teaching Ditto how to transform right and it has become the Pokémon she has always hoped it would be. Team Rocket cries and they give one of the Meowths to Duplica and they leave with the other. Duplica yells that they gave her Meowth and not her Ditto and she chases after them.
Duplica throws Meowth at the Meowth balloon and Ditto bites Jessie's arm and jumps down from the balloon. Duplica catches Ditto and Jessie says its time for Plan B. She pushes a button and a cannon comes out from the bottom of the balloon. Ditto transforms into the cannon and fires Pikachu at Team Rocket, who shocks them terribly. Meowth claws the balloon in pain, causing Team Rocket to go blasting off again.
Ditto transforms back into its regular self and give Duplica a hug. It then transforms into a perfect copy of Pikachu. Ash says Ditto did a great job and asks Duplica which Pikachu is his. Duplica says he needs to keep training.
At the mansion, Duplica puts up a sign that says the House of Imite is reopened. Ash and his friends say goodbye to Duplica and Ditto and they leave. Ash says he wishes he could catch a Ditto because its all of the Pokémon in one.
"Very good, but she dosen't have the adult charm of the real Joy and Jenny." ~Brock after Duplica imitates Nurse Joy and Officer Jenny.
Jessie: "Team Rocket is NOT something to be made fun of, but if you're going to imitate us at least do it RIGHT!"
James: "To 'unite' all peoples within our nation, NOT ignite!"
- This episode marks the first (and only) time Ash and his friends attempted the Team Rocket Motto and completed the whole thing, despite Jessie and James complaining.
- The "Who's that Pokémon?" in this episode is Ditto.