Pokémon : S3 | E20 – Chikorita’s Big Upset

Ash and a Schoolboy are having a Pokémon battle. When Chikorita gets tired, Ash decides to use Pikachu against the Schoolboy's Raticate. When he attempts to return Chikorita into her Poké Ball, Chikorita refuses and evades it. The Schoolboy's mother calls her son in for dinner, just as Ash is finally able to return Chikorita. Ash and his friends go to a Pokémon Center to eat, but Chikorita doesn't participate. Pikachu tries sharing his food with Chikorita, but she bats the plate away with her leaf. Observing this, a glasses-wearing Nurse Joy, who has a degree in Pokémon psychology from Celadon University, walks up and requests having a counseling session with Ash, Pikachu, and Chikorita. After Ash answers several questions, Nurse Joy deduces that Chikorita is jealous of Pikachu. She escorts the group to the Center's greenhouse, and suggests putting Chikorita together with plants for the night to make her feel comfortable.

During the night, Pikachu approaches Chikorita in the greenhouse. As Pikachu watches, Chikorita pops out of her Poké Ball and proceeds to leave the Center. Despite Pikachu's attempts to intervene, Chikorita manages to run off, pursued by Pikachu. She soon finds a warehouse to sleep in, but is awakened by four Fighting-type Pokémon: Machoke, Hitmonlee, Hitmonchan, and Primeape. Chikorita attempts to ignore the Pokémon, but Primeape attacks her. Chikorita soon defeats Primeape with a Vine Whip, shocking the other three and causing them to kneel out of respect for Chikorita.
(Source: Bulbapedia)