Forty years ago, a hunter who specialized in selling rare Pokémon tried to catch a Celebi, a green, winged Pokémon with the ability to travel through time. In order to escape, Celebi used the last of its energy to travel to the present, bringing with it Sammy, a boy who tried to save Celebi from the hunter. In the present, Ash, Pikachu and crew find Sammy in the woods, and an old woman--whom Sammy had met as a girl in the past--helps them all to understand what has transpired. Sammy is determined to help the wounded Celebi recover, and Ash and Pikachu are eager to help. However, they all soon encounter an enemy far more advanced than the hunter Sammy left behind in the past. This new enemy possesses a Pokéball 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. With the help of legendary Pokémon Suicune, will Ash and his friends save the day?