Whilst watching a battle between two powerful monsters, Dialga and Giratina, a Pokémon called Shaymin is thrown into a mysterious realm - Reverse World. He escapes to the real world and meets Ash and the gang, who realise that their journey is similar to Shaymin's. Our heroes join the sharp-witted Pokémon in a dimension-spanning fight to save Reverse World from the diabolical plans of the evil Zero.