Pokémon: The First Movie is the reason Mewtwo is in Detective Pikachu - Polygon

Pokémon: The First Movie: Mewtwo Strikes Back arrived in 1998. More than 20 years later, the story of Ash, Pikachu, Mew, and Mewtwo has new resonance as the Pokémon come to live in live-action while keeping Pokémon game and anime canon alive.

Detective Pikachu, Pokémon Wiki

Pokémon: The First Movie is the reason Mewtwo is in Detective Pikachu - Polygon

Detective Pikachu, Pokémon Wiki

Pokémon Detective Pikachu (2 Years Later) – Jayanth Writes

Pokémon: The First Movie - Mewtwo Strikes Back (1998) - Movie Review : Alternate Ending

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Pokémon: Detective Pikachu's ending and biggest questions, answered - Polygon

Pokemon's first live-action movie 'Detective Pikachu' latest trailer released, Pikachu's intense arrest burst - GIGAZINE

Realistic Pokémon Fanart Got This Artist a Job on Detective Pikachu