Pokemon Best Wishes! - episode 22 - 09 Mar, 2011

Ash, Iris, and Cilan finally arrive in Castelia City and meet Burgh, but they all discover that the city has shut itself down. When they investigate the sewers, they discover and assist a Venipede stuck in a pipe, which, out of fear, poisons Ash. Later, they see a whole swarm of Venipede in the sewers and streets, in which Professor Juniper discovers that the Venipede swarm must have been pushed out of their natural habitat by something. The rest of the group uses their Pokémon to gather the Venipede into Central Plaza to keep the townspeople and the Venipede safe, during which Ash's Pidove evolves into a Tranquil, while Professor Juniper discovers that something is emitting energy in the Desert Resort just as Giovanni heads there to join up with Team Rocket. (Source: Wikipedia)