Pokemon Season 1 Episode 23 - The Tower of Terror (Part 2)

⁣On their way to Lavender Town, the woods suddenly become foggy. The
group quarrels for a bit and arrives at the ghost tower. Team Rocket
encounters the ghost Pokémon while setting a trap for Ash and his
friends, and end up getting frightened by the ghost Pokémon. The gang
hear screaming from the tower and Ash and his friends are afraid to go
in. Haunter turns Ash and Pikachu into ghosts. They learn that it was a
Gastly, Haunter and Gengar looking for playmates. After having fun with
Ash, Haunter decides to follow Ash as his new Pokémon.