Pokemon 4Ever: Celebi - Voice of the Forest

In order to escape a greedy Pokémon hunter, Celebi must use the last of its energy to travel through time to the present day. Celebi brings along Sammy, a boy who had been trying to protect it. Along with Ash, Pikachu, and the rest of the gang, Sammy and Celebi encounter an enemy far more advanced than the hunter left behind in the past. This new enemy possesses a Pokeball called a "Dark Ball," which transforms the Pokémon it captures into evil and far stronger creatures. When Celebi is captured, the fate of the entire forest is threatened. (official distributor synopsis)