It is about 15 years after the story of Pokémon Trainer Red. Red's son lives in Pallet town in the Kanto Region. But Red disappeared somewhere [However Red actually doesn't play that much role in the game]. The protagonist of the game who starts his journey when many things are totally changed in Kanto. After Professor Oak, Brock takes his place as the Pokémon Professor. The protagonist encounters some Pokémon which is not in the Kanto Pokedex. Eventually, he travels through Alola Region! But the main game takes it's place in Kanto.