Ash and friends finally arrive at Castelia City, They find a Venipede only to have a whole swarm invade the city. Meanwhile Giovanni is headed to the Unova region to see the progress Team Rocket has made there. After the swarm passed Professor Juniper arrives and requests the assistance of Ash and co. Ash decides to put his Gym battle with Burgh on hold to help the Professor.
After crossing the Skyarrow bridge, Ash, Iris and Cilan went to Castelia City. Cilan suggested to go to the City's Ice Cream shop, but Ash denies it, wanting to battle the Gym Leader. They managed to find Burgh, thinking of something near a tree. Burgh said that he feels an odd energy, as something was wrong. Iris felt a similar thing and proposed to go to the source. Despite she took them to the Ice Cream shop, they opened a manhole to the sewers.
In the sewers, they find a lone Venipede stuck in a sewer pipe. Despite Burgh warning him not to free Venipede because of the poison, Ash tried (and succeeded) in freeing Venipede. Venipede was struggling to get free, Ash calmed him down, until the poison took effect. Later, Cilan gave Ash antidote, while Burgh was healing Venipede's injury. Suddenly, hundreds of Venipedes were walking the sewers. Ash, Cilan, Iris and Burgh went out.
Unexpectedly, they saw hundreds of Venipedes in the streets! They went to the Pokémon center, when they see Trip and some trainers battling them. Trip almost attacked with his Lampet, until Ash stopped him, blocking the attack. The Mayor comes, declaring that Venipedes must go. The gang persuades him that Venipedes will go to the Castelia City's park until the problem resolves.
The Mayor agreed, Burgh is playing his flute to make the Venipedes follow him. The gang bringed their Pokémon out to lure the other lost Venipedes to follow Burgh. At one point, even Trip joined to help them. When Pidove lured some of the Venipedes, she evolved into Tranquil.
Giovanni, leader of Team Rocket, has been going to visit the source of the unknown energy. Back at the Castelia Town, the gang managed to lure all Venipedes to the park. The Mayor was pleased, while Trip wasn't - he needed to go to another City Gym until things calm down. Professor Juniper arrived, saying that the Venipedes must have gone to the City because there is a secret energy coming from the desert. Officer Jenny volunteered to go with Professor Juniper to the desert to find the secret energy (in the Japanese version, Ash, Iris and Cilan also volunteered to go with Professor Juniper and Officer Jenny). The episode ends with a helicopter going to the mountains.
- Professor Oak's Live Caster: Yamask
- This is the second time Ash looked up Tranquill on the Pokedex. First was BW010: A Rival Battle for Club Champ!.
- Who's That Pokémon?: Venipede (US)
- Although, in Japan, it was shown that [[Ash, Iris and Cilan went with Professor Juniper to the Desert Resort, in the English dub she went alone while Ash and the gang stayed for the next gym badge against Burgh. Basically they skipped the scene in which Ash and co. go in the helicopter due to the next two postponed episodes.