Ash and friends are heading for the Battle Arena, Ash in high hopes of winning his second Battle Frontier Symbol there. However, first, they're heading for Saffron City where May hopes to earn her first Kanto Ribbon. As they walk through the forest, they hang their heads and walk slowly, tired from the journey. May says that they probably already missed it. Up above the trees, Brock and Max spot a Radio Tower, signifying that there is a town nearby. Suddenly, a Caterpie appears and May scans it with her Pokédex. A boy appears on a high tree branch and he accuses them of being the robbers from the previous night. He then orders Caterpie to use String Shot.

All tied up, Ash tells the boy that they aren't robbers. He doesn't believe them and says that they broke into the lab. May and Brock wonder where the lab is as Caterpie uses String Shot again to tie them up more. A man suddenly flies in with a pair of purple wings. He tells them that with an Alakazam's Future Sight, he can read the intentions of humans. He turns on the gadget on his head and tells them that their limo driver is waiting to take them to their own private island. The group tells him that that isn't true and that they are just lost on the way to Saffron City. The boy apologizes to them and they get taken to a lab. Ash is amazed with the place as May comments that they didn't know that the whole forest was the lab. The boy introduces himself as Xander and they all tell him their names.

Max tells Xander that he and Caterpie are awesome. Xander informs everyone that Caterpie is his first Pokémon and that someday they'll go on a journey, but for now they're studying with Doctor Gordon, who is an inventor. Dr. Gordon walks in and says that you can't make an omelet with out first breaking an egg or so they say! He continues that he wants to build a comfortable life for humans and Pokémon alike. From a pool of water, a giant tube appears and the top opens up to reveal a miniature room. Dr. Gordon explains that he built a massage chair with a Magnemite's Thunder Wave and he shows them a karaoke microphone that he built with Jigglypuff's voice. He starts singing into it and quickly falls asleep as Team Rocket spies on them all from outside.

Meowth tells them that they need to steal the inventions for the boss. Jessie tells them that she doesn't want it to be like the night before when Caterpie tied them up with its String Shot. Meowth comments that they'll move in while "the twerps" distract Xander and Dr. Gordon. Outside the lab, Xander and Dr. Gordon bid farewell to Ash and the others as they head out for Saffron once more. Caterpie suddenly races off as Xander wonders if the robbers are back. Dr. Gordon races off to investigate and everyone follows. They run into the lab to find Team Rocket grabbing several blueprints from the shelves as Meowth stores them in a pouch. Dr. Gordon asks if they're Ash and friends, but they say that they aren't. Max alerts him that they're crooks as Xander has Caterpie move in for battle. Jessie sends out Seviper as Meowth moves out with the blueprints.

James grabs an orange box off of the table as Dr. Gordon objects. They start to leave but Caterpie uses String Shot and gets the box back. Seviper steals it back once more, but Pikachu uses Quick Attack and the snake drops it. It bursts open and several small blue balls come bouncing out. Caterpie eats several of them and starts to glow faintly around the edges. It then grows to monstrous proportions. James asks the gargantuan Caterpie if it'd like to talk things over, getting a reply that it doesn't. It attacks them and they go blasting off again, dropping the blueprints in the process. Dr. Gordon tells them that truth is stranger than fiction as Brock explains that the candies were Mystery Candies which enhance a Pokémon?s stats. Dr. Gordon continues that he took every single thing that he could think of that would make a Pokémon better and combined them to make one large vitamin, as Max puts it.

May asks Dr. Gordon if Caterpie will ever return to normal; he tells her that it might, but he hasn't tested it out yet. Caterpie begins to grow even more, and everyone runs for the hills! Outside, Caterpie bursts through the ceiling of the lab and crawls out. It then proceeds to run off towards the town and the Radio Tower. Sirens go off as Caterpie enters town. People run in every which direction as the giant caterpillar slithers through the streets. It breaks through a bridge and continues on.

Meanwhile, Ash, Xander, and the others ride in a cart as Dr. Gordon pulls them from inside a Machoke suit. He explains that he gets 10 times as much energy from the suit while moving about. May puts in that some of his inventions actually work. Xander calls to Caterpie and pleads it to come back, but it looks at the tower mournfully. Dr. Gordon collapses and the cart stops, everyone climbing out. Caterpie begins its ascent of the tower and the top half of it falls off, smashing into the town.

Officer Jenny arrives on the scene as the smoke clears. Dr. Gordon says that it's all his fault. Officer Jenny confirms that he's the one responsible for the Caterpie. All of a sudden, Caterpie begins to glow and evolves into a Metapod. May scans it with her Pokédex as Brock notes that Caterpie knew that it was going to evolve so it climbed to the top of the tower to get bigger and not cause as much damage. Xander tells Metapod that they'll find a way to make it small again and he begs it to come down. Metapod doesn't acquiesce to his request and stays put. On a nearby water tower, Jessie rages that they had something that makes Pokémon bigger and they lost it. Meowth says to imagine the boss with the candies; James says that he?d rather not. Wobbuffet then reveals three more candies in its hand and Jessie swipes them up. She announces to James and Meowth that they're gonna steal the Doctor and have him make them all new inventions. They cheer for promotions as Officer Jenny wonders how to get Metapod small again.

Team Rocket arrives on the scene and says the motto again. Jessie tells Dr. Gordon that he is an unofficial member of Team Rocket as Meowth swipes him with a claw. Pikachu tries a Thunderbolt, but James releases Cacnea and has it use Pin Missile. Pikachu gets hit and retreats; Jessie sends out her Dustox too. She throws the Mystery Candies at Dustox and it eats two of them, letting the third fall to Cacnea, who eats it promptly. Both of the Pokémon grow to oversize and James orders his Cacnea to attack. Cacnea turns around to face James, blushing as it does. It flattens him as Jessie and Meowth get on Dustox with Dr. Gordon. James quickly follows and Dustox takes off, blasting Ash and friends with tons of air as it does.

Up on top of the Radio Tower, Metapod starts to glow and evolves into Butterfree. May scans it with her Pokédex as it lifts off into the air. Xander and Ash get on and give chase to Team Rocket. In the mountains, Dr. Gordon comments about the view and says that he never knew riding on a Dustox could be so pleasant. Meowth tells him that he'll soon be working for them as Butterfree comes into view. Ash yells to Team Rocket to give Dr. Gordon back, but Dustox speeds up, trying to lose them. Xander tells Butterfree to catch it so Jessie commands Dustox to climb and use Poison Sting. Butterfree dodges the Poison Sting and it hurdles towards the ground, where Officer Jenny drives the rest of the gang in her jeep. She avoids them all with some quick steering as Pikachu fires a Thunderbolt at Team Rocket. Dustox dodges it as it and Butterfree come closer to some telephone lines. Pikachu uses Thunderbolt on the steel towers holding them and Dustox is forced to swerve.

Butterfree then uses Supersonic and Dustox gets hit. It suddenly goes back to normal-sized and Team Rocket falls towards the ground with Dr. Gordon next to them. Meowth wonders why Dustox got smaller first when it ate the candies second. Dr. Gordon thinks that it could be a difference in the amount eaten but he isn't sure. Xander and Ash safely catch Dr. Gordon on Butterfree and let Team Rocket fall towards a lake. They crawl out of the lake to find May and the others arriving with Officer Jenny. Jenny tells them that they are under arrest for theft and attempted kidnapping. Jessie orders Dustox to use Psybeam. Pikachu surprises it by using a Thunder from behind as Butterfree brings Ash, Dr. Gordon, and Xander in. With a Whirlwind from Butterfree, Team Rocket is sent blasting off again.

That night, as the sun sets on the lake and the surrounding area, Butterfree shrinks back to its normal size and flies down to Xander. May comments that it was cool to see a giant Butterfree, but she's glad it's back to normal. Officer Jenny tells Dr. Gordon to lock away all of his Mystery Candy so that it doesn't happen again, to which the Dr. replies that he has none left. He says that it was just a mistake and that failure is a step on the road to success. He continues that he'll continue to make great inventions. Officer Jenny tells him to keep them inside his lab next time. Xander then asks Butterfree if it would like to be his Pokémon and accompany him on his Pokémon Journey. Butterfree obliges and Ash says that maybe they could battle someday. Xander says that he?ll have to do a lot of training before he can battle Ash. May chimes in that she's really psyched for her next Contest in Saffron City. And now the gang knows that candy isn't always good for you, bigger is not always better, and trying your best is as good as achieving success.

English Official Summary

On their way to Saffron City, Ash and his friends encounter a Caterpie and its Trainer-in-training, Xander, who mistakes them for lab robbers. Fortunately, Dr. Gordon, the lab owner and Xander's teacher, shows up and things are sorted out. The kids visit Dr. Gordon's lab, where he makes all kinds of strange inventions. Of course, Team Rocket wants to steal them all! Caterpie helps catch them in the act, but in the process, Dr. Gordon's briefcase opens and blue candies roll out. Caterpie eats some and grows until it's big enough to send Team Rocket blasting off. The Mystery Candy Complete keeps Caterpie growing until it's bigger than a building! The giant Caterpie heads into town as people flee in a panic. Climbing the local radio tower, it then evolves into Metapod. Elsewhere, Team Rocket discovers that Wobbuffet stole some Mystery Candy too, so they head back into town and kidnap Dr. Gordon. The Candy makes Dustox and Cacnea huge—Dustox is so big that Team Rocket hops on its back and flies away with Dr. Gordon! Metapod evolves into a giant Butterfree just in time for Ash and Xander to give chase. After some aerial maneuvers, Dustox suddenly shrinks back to normal size. Butterfree catches it but Team Rocket plummets into a pond. No sooner do they emerge than Officer Jenny arrives with Ash's friends, and when Team Rocket puts up a fight, Pikachu and Butterfree send them on their way once more. Eventually Butterfree returns to normal size too, and Ash says farewell to Xander and his newly-evolved friend—as well as Dr. Gordon, who's now more enthusiastic about his inventions than ever!

French Official Summary

Sur le chemin de Safrania, Sacha et ses amis rencontrent un Chenipan et son apprenti Dresseur, Xander, qui les prend pour des voleurs de laboratoire.

German Official Summary

Auf dem Weg nach Saffronia City treffen Ash und Gefährten auf ein Raupy und seinen momentan noch in Ausbildung befindlichen Trainer Xander, der sie für Diebe hält.

Italian Official Summary

Sulla strada per Zafferanopoli, Ash e i suoi amici incontrano un Caterpie e il suo aspirante Allenatore, Xander, che li crede i ladri che nei giorni precedenti si sono infiltrati nel laboratorio.

Portuguese Official Summary

Depois de um experimento científico dar errado, Ash e seus amigos tentam rastrear um Caterpie  que cresceu em um tamanho desproporcional!

Finnish Official Summary

Tieteellisen kokeen epäonnistuttua Ash ystävineen yrittää tavoittaa Caterpieta, joka on kasvanut jättimäiseksi!

Spanish Latin America Official Summary

Después de que un experimento científico sale mal, Ash y sus amigos tratan de rastrear a un Caterpie que ha crecido hasta enormes proporciones.

Spanish Official Summary

De camino a Ciudad Azafrán, Ash y sus amigos se encuentran un Caterpie y a Xander,su Entrenador que está entrenándole, y quien los confunde con ladrones de laboratorio.

English Great Britian Official Summary

On their way to Saffron City, Ash and his friends encounter a Caterpie and its Trainer-in-training, Xander, who mistakes them for lab robbers. Fortunately, Dr. Gordon, the lab owner and Xander's teacher, shows up and things are sorted out. The kids visit Dr. Gordon's lab, where he makes all kinds of strange inventions. Of course, Team Rocket wants to steal them all! Caterpie helps catch them in the act, but in the process, Dr. Gordon's briefcase opens and blue candies roll out. Caterpie eats some and grows until it's big enough to send Team Rocket blasting off. The Mystery Candy Complete keeps Caterpie growing until it's bigger than a building! The giant Caterpie heads into town as people flee in a panic. Climbing the local radio tower, it then evolves into Metapod. Elsewhere, Team Rocket discovers that Wobbuffet stole some Mystery Candy too, so they head back into town and kidnap Dr. Gordon. The Candy makes Dustox and Cacnea huge—Dustox is so big that Team Rocket hops on its back and flies away with Dr. Gordon! Metapod evolves into a giant Butterfree just in time for Ash and Xander to give chase. After some aerial maneuvers, Dustox suddenly shrinks back to normal size. Butterfree catches it but Team Rocket plummets into a pond. No sooner do they emerge than Officer Jenny arrives with Ash's friends, and when Team Rocket puts up a fight, Pikachu and Butterfree send them on their way once more. Eventually Butterfree returns to normal size too, and Ash says farewell to Xander and his newly-evolved friend—as well as Dr. Gordon, who's now more enthusiastic about his inventions than ever!

Russian Official Summary

Когда Команда Р нападает на лабораторию изобретателя, его Катерпи случайно съедает экспериментальный продукт и начинает увеличиваться в размерах. Не желая причинить зла местным жителям, Катерпи уходит их города и эволюционирует в Метапода. Но в планах Команды Р использовать то же вещество, чтобы увеличить Дастокса, и использовать его мощь и силу в своих коварных планах.

Dutch Official Summary

Nadat een wetenschappelijk experiment mislukt, proberen Ash en zijn vrienden een Caterpie te vinden die uitgegroeid is tot enorme proporties!

Norwegian Official Summary

Etter at et vitenskaplig eksperiment går galt, prøver Ash og vennene hans å finne en Caterpie som har vokst til enorme proporsjoner!

Swedish Official Summary

Efter att ett vetenskapligt experiment gått snett, försöker Ash och hans vänner spåra upp en Caterpie som har växt till enorma proportioner!

Danish Official Summary

Da et videnskabeligt eksperiment er gået helt skævt, må Ash og hans venner forsøge at opspore en Caterpie, som er blevet helt usandsynligt stor!