Mewtwo is the main opponent in Pokémon: The First Movie.
The film, released in Japan in 1998 and internationally as Pokémon: The First Movie in 1999, centers on Mewtwo’s origin and conflict with humanity. Scientists create Mewtwo from genetic material connected to the Mythical Pokémon Mew, giving Mewtwo extraordinary psychic abilities.
Mewtwo initially believes humans have used and abandoned it. It therefore challenges Pokémon Trainers and attempts to prove that artificially created Pokémon can surpass naturally born Pokémon. Ash, Misty, Brock, and their Pokémon become involved after receiving an invitation to a mysterious island.
Mew is important to the story but is not the principal opponent. It appears as the genetic source connected to Mewtwo and ultimately becomes part of the film’s climactic conflict. The movie’s subtitle in Japan was Mewtwo Strikes Back, while the international title emphasized that it was the first Pokémon feature film.