Pokémon: Giratina and the Sky Warrior

The Reverse World is a world that accepts pollution from the Real World in the form of poison so that the environmental balance of the Real World isn't destroyed by air pollution. Giratina looks upon Dialga and Palkia as its enemies, because the effects of contamination in the Reverse World have increased as a consequence of last year's time-space clash. Dialga, who is resting at a lake, is attacked from the Reverse World by Giratina and dragged to the Reverse World, and Shaymin, who is at the scene, also gets caught up. Escape is impossible for Dialga, but Shaymin manages to escape by absorbing the poison that's all over the Reverse World and exploding. When it does, it opens a hole in the Reverse World, and Dialga and Shaymin escape. While escaping, Dialga places the Reverse World in a time loop, closing it so Giratina can't enter the Real World.

Synopsis from Anime News Network