Pikachu's Rescue Adventure - S0.E5

Ash, Misty, and Tracey deciding bringout all of their Pokémon, and the go off somewhere. While sleeping, Togepi falls down a steep hill and disappears into a hole. Pikachu, Bulbasaur, Squirtle, Marill, Venonat and Psyduck follow Togepi. They end up in a giant tree where Pikachu appears to have lived in before.