The original Pokémon games are set in the region of Kanto.
Kanto contains locations such as Pallet Town, Viridian City, Pewter City, and the Pokémon League. Players travel through the region while collecting Pokémon, defeating eight Gym Leaders, and challenging the Elite Four.
Interestingly, the name Kanto was not used in the original Japanese releases of Pokémon Red and Green. The region received that name in the second generation, when Pokémon Gold and Silver allowed players to return to the original game world.
Kanto is based partly on Japan’s real Kantō region, which includes Tokyo and surrounding areas. It is the setting associated with the first 151 Pokémon, including Pikachu, Charizard, and Mewtwo.