The gang is a bit tired from all the journey to Saffron City. Max spots a radio tower, but a Caterpie appears. A boy comes, accusing them for robbery, so Caterpie uses String Shot, immobilizing the gang. He accuses them for the robbery inside the lab and a scientist comes with a jet pack. He introduces himself as Dr. Gordon and using the machine on its head (which works as Future Sight), determines that a limo driver is waiting for them, so they say they are just to Saffron City.
The boy apologizes, but Ash says they did not know that the entire forest was a lab. The boy is Xander and the gang introduces themselves. Ash tells Caterpie is cool, so Xander tells him it is his first Pokémon. He is here to study to become a trainer, under the tutelage of Dr. Gordon. Dr. Gordon tells that he makes inventions to make the lives of humans and Pokémon easier. One of his inventions is a microphone - he sings and the Jigglypuff on the microphone reproduces the sound as it was using Sing, so Gordon falls to sleep. Team Rocket spots the machines and think they should get the blueprints, but Jessie reminds them that Caterpie used on them String Shot last night, so James and Meowth tell her that this time they'll sneak in and take the blueprints while the others are distracted.
Dr. Gordon shows them the way. Gordon and Xander say goodbye, but Caterpie senses something and goes inside the lab. Team Rocket steals the plans, but everyone arrives. Jessie sends Seviper, but Meowth tells everyone to hurry up. James grabs a suitcase, in which Gordon's greatest invention he made is. However, Caterpie grabs the suitcase using String Shot. Still, Seviper gets the suitcase back, so Pikachu uses Quick Attack. Some berries come out of the suitcase and Caterpie eats them, becoming larger and larger. Caterpie tackles Team Rocket, blasting them off, though the plans are left inside the lab. Gordon tells Xander the berries Caterpie ate caused it to have its abilities and strength enhanced. Gordon does not know if Caterpie will go back to its original state, but will try to size it back, though Caterpie grows larger.
May tells Xander to call Caterpie back into the Poké Ball, but Xander did not catch it. Caterpie becomes so large it destroys the roof. Caterpie goes out and heads for the city itself. Gordon runs, pulling a wagon with the gang inside, being in a Machoke suit to increase his strength and speed. They arrive to the radio tower, but Caterpie goes onto the tower. Because of its size and weight, Caterpie brings down a part of the tower. Officer Jenny comes, with Gordon telling it is all his fault Caterpie got so big. Caterpie begins to glow and evolves to Metapod. Brock tells them it wanted to be on a safer spot and be prepared to evolve into Butterfree, because it wants to deal less damage. Xander tells Metapod to come down to help it, but Metapod falls asleep.
Team Rocket witnesses this and wish they had the berries. Surprisingly, Wobbuffet gives them the berries. They plan to kidnap the Dr. Gordon and take it to the Headquarters, meaning a big promotion for them. Jenny asks Gordon how to deal with it, so Gordon thinks, but Team Rocket come and tell Gordon is now a member of Team Rocket. They use a handcuff to get Gordon to them. Pikachu goes to use Thunderbolt, though James sends Cancea, who hits Pikachu with a Pin Missile. Jessie sends Dustox and eats the berries, but accidentaly Cacnea eats one as well. James orders the big Cacnea to attack, but Cancea turns and hugs him, crushing him to the ground. Team Rocket go on Dustox and fly off. Suddenly, Metapod evolves into Butterfree. So, Ash and Xander go on it to defeat Team Rocket.
Gordon remarks the view and James spots the giant Butterfree. Dustox flies away, but Butterfree speeds up. Dustox goes up and uses Poison Sting, but Butterfree and the rest of the gang in Officer Jenny's car evade the missiles. Pikachu uses Thunderbolt, but Dustox moves away. Pikachu tries again, but this time on the steel tower, electrocuting the wires, so Dustox loses its balance in fear. Butterfree uses Silver Wind, confusing Team Rocket and Dustox. Dustox reverts back to its original size, so Team Rocket falls down, but Butterfree catches Gordon, while Team Rocket fall into a lake. Team Rocket goes to the coast and Jenny arrests them for theft, but Dustox uses Psi Beam, but the attack is canceled by Pikachu's Thunder and Butterfree blasts them off with Whirlwind.
At the end of the day, Butterfree reverts to its original size. Jenny tells Dr. Gordon to lock the berries, but Gordon tells them they were a failure. Xander tells Butterfree they will train, so Ash thinks they might meet each other again and then battle, while May plans to enter the Saffron City's Contest.
- Featured Pokémon: Sableye, Sitty, Nincada
- This is the last episode in which Jamie Davyous Owens voices Officer Jenny.
- In the original version, the candy that Dr. Gordon invents (which Xander later gives to his Caterpie, causing it to become Giant) is Rare Candy (Fushigina Ame). The dub mistakenly calls it "Mystery Candy," which is a literal translation of its Japanese name.