A Venipede Stampede

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 a Venipede stuck in a pipe, which they help out but because of its fear it poisoned Ash. Later, they see a whole swarm of Venipede in the sewers which flood the streets. Trainers with Fire Pokémon do their best to fight the swarm, including Trip and his newly captured Lampent. However, Ash tries to stop them from hurting the Venipede, as hurting them will not get rid of them. Professor Juniper studies the Venipede swarm back in Nuvema Town, realizing they must have been pushed out of their natural habitat by something. Burgh tries to figure out the problem in Castelia City when Trip continues to use Lampent to get rid of the Venipede, Ash being struck by a Shadow Ball in the process. Nurse Joy arrives with her Audino calm down several of the Venipede, and the Venipede which they helped earlier came and helped to calm them. The rest of the group uses their Pokémon to gather the Venipede into Central Plaza to keep the townspeople and the Venipede safe. During this, 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. As the mayor thanks Burgh, Ash, Iris, Cilan, and Trip for helping, Professor Juniper flies in and tells everyone what is happening in the Desert Resort. Ash, Iris, and Cilan head out with her to the desert as Burgh promises he will let Ash challenge him for the Insect Badge when everything is cleared up.

Source(s): Wikipedia