In the first Pokémon film, Mewtwo's past is briefly mentioned, but at one point it plays a significantly larger role in the narrative.
Millennials recall Mewtwo's goodbye remarks from the first Pokémon film. Many fans have never seen a key scene in that movie that explains Mewtwo's behavior.
For the English dub, Pokémon The First Movie: Mewtwo Strikes Back is longer than 75 minutes. Fans call it the "kanzenban" version, which adds 10 minutes of Mewtwo's pre-escape footage and additional graphics.
The English film included those adjustments, but the extra 10 minutes were reduced to match the Japanese. Although unused, certain sequences were dubbed.
The DVD special, "The Uncut Story of Mewtwo's Origin," used the added info. Dr. Fuji returns the Mew fossil to his lab. Mewtwo, Charmander, Squirtle, Bulbasaur, and Amber—Doctor Fuji's clones—are in the tube.
Amber instructs clones to telepathically speak with Mewtwo. Amber alone kills clones after instability. They're alive and well, she informs Mewtwo before leaving. Losing everything, Mewtwo remembers Amber vaguely.
In the theatrical version, adult Mewtwo destroys the lab and heads to his island fortress. As Ash and other trainers arrive on the island, Mewtwo has time to repair Dr. Fuji's cloning technology.
Mewtwo created Charizard, Venusaur, and Blastoise clones from his forgotten comrades. Ash is petrified at the end of the film, but Amber remarks "Pokémon's tears are full of life," predicting his rescue.
Mewtwo's "The gift of life" quote reflects Amber's short departure. A Japanese-only radio play inspired the extra scenes.
Mewtwo's past makes him more than a sad Pokémon experiment to win. These scenes help it live up to its reputation, even though many theater-only Pokémon enthusiasts never watch it.
Also See
Only One Legendary Pokémon Was Designed For A Movie, Not the Games