Pokémon S23E01 - Enter Pikachu!

In this prequel episode, Pikachu's backstory as a Pichu, his being cared for by a Kangaskhan after falling off a cliff, and Ash's backstory when he was 6 years old missing Professor Oak's camp because he overslept and breaking the Poké Ball Clock, the events leading to his evolution into a Pikachu and eventually meeting Ash are revealed.