Mewtwo was created by humans 20 years before but he managed to escape. Months before the events of the film, Harry Goodman was recruited by Howard Clifford to track down Mewtwo, which eventually led to his capture. Trapped in a spherical chamber, Mewtwo was used by Dr. Ann Laurent to produce the R substance, an aerosolized drug that can make Pokémon go on a rampage. However, Mewtwo was freed by Harry and his partner Pikachu, making Mewtwo realizing that not all humans are evil. When Harry's car crashed in an ambush, Mewtwo came to his rescue and combined a dying Harry into Pikachu's body but also erased his memory, creating Detective Pikachu as a result.
When Detective Pikachu was critically injured, a bunch of Bulbasaur brought Tim and Pikachu to Mewtwo's location, where he healed Pikachu. But before Mewtwo was able to fully reveal the secret of Harry's case, he was captured and taken away by Howard's man. When Tim arrived at Howard's office, he learnt that Howard was trying to transfer his mind into Mewtwo, so that he could use Mewtwo's ability to transfer all humans' minds into their Pokémon. Mewtwo tried to warn Tim to run before his body was taken over.
After a long struggle, Tim was able to remove Howard's neural link and Mewtwo was freed. As Detective Pikachu was about to fall to his death, Mewtwo used his psychic powers to save him. He then reversed the disaster caused by Howard. Before taking his leave, Mewtwo revealed that Detective Pikachu was actually Tim's father Harry Goodman and unmerged Harry out of Pikachu's body.
Personality and characteristics
This Mewtwo is portrayed as a benevolent figure despite his mistrust of humans, much like bothMewtwo in the anime after their character development. While mostly viewed by Tim and Pikachu as a dangerous, malevolent figure prior to meeting him, Mewtwo is in fact a protector of both humans and Pokémon, able to use his abilities to save the lives of others as he watches over them.
This Mewtwo differs from previous depictions of the character in two ways. The first is that he possesses the unique ability to transfer the minds of humans into Pokémon. This ability could only be done when the receiving Pokémon are willing (as in the case of Detective Pikachu) or when they lose control of themselves (when drugged with the R substance). The second is that he speaks with an androgynous "fused" telepathic voice provided by a male and a female voice actor; despite this, unlike previous Mewtwo, which were almost always referred to with gender-neutral pronouns, he is referred to with male pronouns throughout the movie. However, when Howard's mind is transferred into Mewtwo, he speaks non-telepathically with Howard's voice.
Note: Mewtwo is voiced by both actors using a fusion effect.