After winding up in the middle of the battle between Giratina and Dialga, Shaymin somehow reached the place where Satoshi (Ash) and his friends are taking a break...
But with the battle going on in the Torn World, also known as the Reverse World, they somehow got involved, leading to a more complicated case than they ever imagined...

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Parent story: Pocket Monsters: Diamond and Pearl
Prequel: Gekijouban Pocket Monsters Diamond & Pearl: Dialga vs. Palkia vs. Darkrai
Sequel: Gekijouban Pocket Monsters Diamond & Pearl: Arceus - Choukoku no Jikuu e