In a forest somewhere in Kanto, there once was a lonely Pichu. This Pichu, who was still very young, went through a certain encounter and farewell. This all took place shortly before Satoshi and his partner Pikachu started their journey.
One day, Pichu fell off a cliff and was rescued by a Kangaskhan who placed Pichu in her pouch. After time, Pichu began to get bigger and older and one night, it left the Kangaskhan pack and evolved into a Pikachu.