Dans un monde où les gens ramassent des petits monstres (les Pokémons) pour les amener à se battre, un garçon tombe sur une créature intelligente qui cherche à devenir un détective.
Detective Pikachu wakes up in the middle of nowhere with amnesia and the only clue he has to his past is a name and address written inside his cap. Pikachu makes his way to the address in Ryme City and finds Tim Goodman , who's astonished to find the little yellow creature can talk. The two of them team up to find Tim's missing father, Harry. Chasing clues through the streets of Ryme City, the dynamic duo soon discover a devious plot that poses a threat to the Pokémon universe.