Celebi and Joy! - S1.E17

Richie and Sparky are in the big city, where the nearest Pokémon Center is scheduled for demolition. Two Nurse Joys—a grandmother and granddaughter—are trying to stop the demolition crew, so Richie and his Pokémon step in to drive off the crew's Pokémon. The Nurse Joys tell him that this Pokémon Center opened 75 years ago, back when the area was a rural place where Celebi lived. That was the same day the elder Nurse Joy's friend, Nick, was lost in a landslide. That night, a Celebi appears before Richie and shows him a Celebi carving hidden in the Pokémon Center's furniture. Then Richie is surrounded by light, and the next thing he knows, he's in the middle of a forest!

Richie hears a cry for help, and he rescues a young Joy from falling off a cliff. Joy leads him to the local Celebi shrine, where he's introduced to her friend Nick. Richie is in the past—and it's the day before the Pokémon Center first opened!