While trying to escape from a Pokémon Hunter, Celebi stumbled across Sam, who was out looking for Pokémon. Just as the hunter was about to capture the Pokémon, it time traveled with Samuel Oak to escape and ended up in the time of Ash and his friends.
However, there was still danger in the present, this time from a Team Rocket member known as the Iron-Masked Marauder. Using a Dark Ball, the Iron-Masked Marauder captured Celebi to use for his own evil plans, and the Pokemon was put under his dark control. With the help of Ash and the others, Celebi was set free from the Iron-Masked Marauder. However, it looked very grim for Celebi, who wasn't able to heal itself in the Lake of Life where it had healed before because the lake was too polluted.
Fortunately, many other Celebi from the past and future came to heal their friend. Once Celebi was healed, it was able to go back to the past and take Sammy with it.
Kazuko Sugiyama (Japanese)