Pokémon - Season 7 Episode 49 A Scare to Remember! Gotta catch 'em all!
Overview: After another encounter with Team Rocket, Ash loses Pikachu. Pikachu, however, has hit his head and suffers from amnesia and is found by Meowth. Meowth takes the opportunity and states Pikachu was one of Team Rocket members. Pikachu starts believing and once Ash comes to him, Pikachu turns his back on him. Ash, however, does not give up on Pikachu and tries to find a way to bring his memories back.