Pokemon: Pikachu no Natsuyasumi
English name: Pokémon: Pikachu's Summer Vacation
Genres: Comedy, Fantasy
Type: Movie, Episodes: 1
Aired: Jul 18, 1998
Studio: OLM
Source: Game
Rating: PG - Children
Synopsis:
Ash and his friends arrive at a Pokémon-only park. They set all their Pokémon loose and, after Ash tells Pikachu to watch over Togepi, they enter for a day of fun and relaxation. Unfortunately, it's not going to turn out the way they had hoped.

(Source: Serebii)