Pikachu no Fushigi na Fushigi na Daibouken — Episode 1

While being let out to play by their trainers, Pikachu and Piplup wind up separated from their Pokémon friends and wind up travelling with a Skitty into a strange dimension, unknowingly followed by Croagunk, Team Rocket's Pokémon and a Marill that's desperate to have someone hear her sing... (Source: Bulbapedia)