The official Pokemon movie website has updated with a feature on the upcoming movie tie-in game The Thieves and the 1000 Pokémon (とうぞくと1000びきのポケモン). Translations follow:


Free to play! A new Pokémon game based on "The Cocoon of Destruction and Diancie"!

A new Pokémon game utilizing the Streetpass system!
Streetpass people and keep incrasing your Pokémon!
"The Thieves and the 1000 Pokémon" will be available from Thursday June 5th!
*This game is exclusive to the Nintendo 3DS download service. It will be available from Thursday June 5th to Tuesday September 30th


The same setting as the movie!
Incredibly, the story is connected to this year's movie. You will go up against Marilyn Flame and Ninja Riot!
"The Thieves and the 1000 Pokémon" is set in "Diamond Ore Country", just like this year's movie. The thieves Marilyn and Riot also appear in the movie. What are they planning to do? How does it relate to the movie's plot?
Marilyn
In the movie, she has a Mafoxy as her partner. What kind of Pokémon will she use in the game?
Riot
Despite being called a ninja thief, he's quite tough.

It connects with the movie!
If you download "The Thieves and the 1000 Pokémon" and then go see the movie in a theater, "something special" will happen!? This can then be further connected to "Pocket Monsters X/Y"!
With The Thieves and the 1000 Pokémon, at a theater showing the Pokémon movie: Download "The Thieves and the 1000 Pokémon" and go to a theater to watch "Pokémon the Movie XY: The Cocoon of Destruction and Diancie" to make "something special" happen!
With Pocket Monsters X/Y: Challenge that "special something", and something amazing will happen in "Pocket Monsters X/Y"!


How to play
"The Thieves and the 1000 Pokémon" is a game you play using the Nintendo 3DS Streetpass function! When you streetpass users that have the same game, you will gain more Harimaron, Fokko and Keromatsu! With a band of Pokémon in tow, battle the thieves that stand in your way! Streetpass a lot, gain a lot of Pokémon, and beat the thieves!
You assemble 1000 Pokémon and go for victory too!

For further information on how to play, how to download the game, etc, see the official Pocket Monsters website.

Trailer

Source: http://www.pokemon-movie.jp/topics/thief_and_1000pokemon.php