Chikorita's Big Upset

During a tough Pokemon Battle, Ash tries to call back Chikorita in order to use Pikachu, but Chikorita refuses to go back. Ash continues to try, but eventually the boy has to leave. At the Pokemon Center, Professor Oak and Nurse Joy tell Ash that Chikorita is probably jealous of Pikachu. That night Chikorita sneaks out of the Pokemon Center and Pikachu follows, but what will happen to the two?