When Ash seems fixated on training Riolu, Pikachu, feeling left out and jealous, decides to run away back to Pallet Town. Mimey chases after him, but Pikachu won't listen, so Mimey determinedly follows him. This begins a journey in which not only are memories revived and re-experienced, but also both new and old bonds are formed and re-forged. (Source: Wikipedia)
