Episode 4 - The Keystone Pops!

After Ash's Buizel destroys the Hallowed Tower, a Spiritomb is unleashed and attacks Ash, Dawn, Brock and a nearby village. Then Jessie tells it that Ash is a descendant of the Aura Guardian that coincidentally has a Pikachu, that sealed it away. So Ash sets out to stop the Spiritomb and with Pikachu's help, they seal it away in the Hallowed Tower.