Mewtwo Strikes Back: The Birth of Mewtwo

Sadly, about 10 minutes of the film were not shown outside Japan, and these 10 minutes showed how Mewtwo was born. He made friends with Ai,the daughter of one of the Professor's daughters. Maybe it was left out because Pokémon and Ai die in this bit, but I don't know.
In Japan there was a CD called "The Birth of Mewtwo" where there was even more information than in the 10 minutes that where left out.

Story in Pictures | Story from the CD |