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 Pok mon friends and wind up travelling with a Skitty into a strange dimension, unknowingly followed by Croagunk, Team Rocket's Pok mon and a Marill that's desperate to have someone hear her sing... (Source: Bulbapedia)