Pokémon: The First Movie - Mewtwo Strikes Back

Ash Ketchum and friends are tricked into visiting New Island, an island ruled by a mutant Pokémon, Mewtwo. There, they find out about Mewtwo's plans to clone their Pokémon and destroy the world, and a massive battle follows between the clones and the originals - including the "original" counterpart to Mewtwo: Mew.