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The premise of The Thieves and the 1,000 Pokémon is to befriend and collect Pokémon through StreetPass. You can use Pokémon to take on the band of thieves and their Pokémon by clashing in stampede-style battles to retrieve stolen treasure.
If you don't have enough Pokémon, you'll end up losing against the thieves, so you'll actually need to have quite a lot of Pokémon to get far in the game. The villains are characters who'll be appearing in the upcoming film, Pokémon the Movie: Diancie and the Cocoon of Destruction.
The trailer also mentions that there's a special something in store for those who play the game and take it to the movie theaters when the new Pokémon movie premieres. There will also be something special for those with Pokémon X and Y.