Pokemon Season 1 Episode 20 - The Ghost of Maiden's Peak

⁣The boat that Ash and his friends were in lands at a small town. This
town is celebrating Obon. Brock and James both see a girl standing by
the sea who disappears, and while Brock falls for this mirage girl, Ash,
Misty and Team Rocket have a different plan. They try to stop Brock and
James and suddenly, the maiden reveals herself as Gastly, a ghost
Pokémon. It is revealed that the maiden actually exists as a ghost and
is still waiting for the one she loves to return. When Ash battles the
Gastly with Squirtle/Bulbasaur, Gastly turns into Venustoise.