Complex sentence: It is only after Ash protects Pikachu from a group of angry Pokémon that Pikachu realizes how much Ash cares for him, and he saves Ash, and they both see a mysterious unidentifiable Pokémon that spurs both of them to work towards Ash's goal.
Split into simpler sentences: It is only after Ash protects Pikachu from a group of angry Pokémon that Pikachu realizes how much Ash cares for him, leading him to save Ash. Afterward, they both see a mysterious unidentifiable Pokémon that spurs both of them to work towards Ash's goal.