In the show, Mewtwo was genetically engineered by Team Rocket's leader, Giovanni, to be the ultimate Pokemon. He was created using DNA from 151st Pokemon, Mew. When Mewtwo became aware that he was no more than a puppet, he devastated Team Rocket's base and escaped. He has since created his own genetically altered Pokemon and held a tournament in Mewtwo Strikes Back, the first Pokemon movie.
Mewtwo is also one of the secret characters in Super Smash Bros. Melee. To unlock Mewtwo in Super Smash Bros. Melee, you must play 700 matches of Versus mode or play 20 total hours of Versus mode.