Pikachu no Fushigi na Fushigi na Daibouken

Synopsis

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)