In Pokémon: The First Movie, Mewtwo was created through genetic research involving Mew.
The film's story presents Mewtwo as a genetically engineered clone derived from Mew. Scientists working under Team Rocket leader Giovanni attempt to create a powerful Pokémon, but the resulting being develops its own intelligence, identity, and resentment toward the humans who treated it as an experiment.
Mewtwo's origin gives the film its central conflict. It questions whether a created being is defined by its origins, its abilities, or the choices it makes. The film then builds its action around Mewtwo's challenge to trainers and its confrontation with Mew.
Mew and Mewtwo are often confused because their names and designs are related. Mew is the ancient Mythical Pokémon used as the genetic source in the story, while Mewtwo is the separate engineered creature created from that research. Ditto has cloning-related associations in fan discussions but is not the answer to this film question.