Ash receives Pikachu from Professor Oak in the Pokémon anime premiere.
In “Pokémon, I Choose You!”, Ash arrives late at Professor Oak's laboratory and misses the chance to select Bulbasaur, Charmander, or Squirtle. Oak gives him Pikachu instead, making Pikachu Ash's first official Pokémon partner.
Pikachu initially refuses to cooperate and dislikes staying inside its Poké Ball. Their relationship changes when Ash risks himself to protect Pikachu from attacking Spearow, after which Pikachu uses its electrical power to defend him.
The episode established Pikachu as the anime's central mascot. In the video games, players normally begin with one of three Kanto starter Pokémon, so the anime's choice deliberately differs from the original game opening.