Ditto's Mysterious Mansion

S1, E37: Caught in a sudden rain shower, our heroes quickly try to seek some shelter. In the middle of a clearing, they find a small, ghastly cabin. Dragging a very reluctant Misty, Ash and Brock go inside the cabin, and instantly get the impression that they are not alone! Soon enough, Pikachu discovers that another Pikachu is there with them, although this one has a slightly odd appearance. When the Trainer Duplica arrives, all is revealed: her Pikachu is in fact a Ditto, the Transform Pokémon, and our heroes are in her Copycat House. Intrigued by Ditto’s Transform move, Ash proposes a battle between Ditto and his Bulbasaur!