Pokémon The First Movie: Mewtwo Strikes Back (G)
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This is, as the title suggests, the first movie based on the Pokémon TV series. Before the main movie is the short film Pikachu's Vacation.

In the main movie, Giovanni, Team Rocket's boss, had a bunch of scientists use legendary pokemon Mew's DNA to create a clone, Mewtwo, who was the most powerful pokemon in the world. But Mewtwo didn't want to be used by humans, so he set out to take over the world himself. He lured trainers, including Ash and company, to New Island, where he cloned all their pokemon. Then there's a big battle and stuff, and everyone learns a valuable lesson in the end. Although ultimately it was rather forgettable. Anyway, the movie wasn't great, but I liked it. I should probably try to see it again someday, and write a better review.