Caterpie's Big Dilemma (Japanese: 進化！その神秘と奇跡！！Evolution! That Mystery and Wonder!!) is the 142nd episode of the Advanced Generation series, and the 416th episode of the Pokémon anime. It was first broadcast in Japan on September 8, 2005 and in the United States on June 10, 2006.
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!
As Ash and his friends head towards Saffron City, an angry Caterpie suddenly pops out from the bushes. Curious about it, May brings out her Pokédex. A young boy then appears and accuses the group of being thieves. The boy commands Caterpie to use String Shot on Ash and the gang. Ash and friends cry out in protest, but Caterpie continues to encase them in String Shot. A man in a lab coat called Dr. Gordon and with a strange device on his head comes onto the scene, and tries to determine the truth about the trio. Using the device's Alakazam, he determines that their limousine is waiting for them. Eventually, the group convinces Dr. Gordon and the boy Xander that they are just travelers. Xander apologizes for accosting them, before everyone introduces each other.
Ash compliment's Xander's Caterpie for its strong String Shot. Xander mentions that the Caterpie is his first Pokémon and his best friend and that they plan to go on a journey soon. Dr. Gordon then appears and raises a strange contraption from a nearby pool of water. It rises to the top and opens up, revealing an array of inventions based on different Pokémon's abilities, according to the doctor. He grabs one device, a microphone with a Jigglypuff figurine on top of it, and starts singing into it. The Jigglypuff figure starts singing its melody as well. The doctor quickly falls fast asleep.
Outside, Jessie and James observe. Jessie brings up the idea of capturing Caterpie, especially after it ruined their plans last night and made them flee while encased in String Shot.
As the doctor wishes them luck towards their Pokémon Contest, Caterpie senses something and heads down the trail. Jessie, James, and Meowth are rummaging through the doctor's belongings as Caterpie appears in the doorway. Everyone else appears soon afterward, and a Pokémon battle is about to take place. Caterpie heads into battle as Jessie calls upon her Seviper. When James picks up an orange case, Dr. Gordon becomes very concerned about what he calls his most important invention. Caterpie snatches the case with its String Shot, but Seviper grabs hold of it with its fangs, severing the strand. Pikachu delivers a swift Quick Attack into Seviper, causing the case to fall to the ground and open, revealing several small, blue balls.
Caterpie then eats one and a reddish light appears around its body. Caterpie's body increases to several times its normal size, reaching all the way to the lab ceiling. Caterpie Tackles Team Rocket and sends them blasting off and leaving the stolen objects behind. Brock figures that the blue balls are "Mystery Candy," but the doctor says that it's his "Mystery Candy Complete." The balls are meant to be a multivitamin to maximize a Pokémon's potential, but the doctor had no idea that this is what would happen. The mysterious light appears again as Caterpie then grows in size even more and breaks through the ceiling. May tells Xander to return it to its Poké Ball, but he reveals that he has never actually caught Caterpie.
Caterpie heads towards the nearby city, stampeding through and breaking right through a bridge. The doctor dons one of his inventions, a Machoke suit that increases one's strength by a factor of ten, and pulls a wagon containing the others. Caterpie then arrives at a huge tower in the city and starts to climb it. About halfway up, the top half of the tower fails due to Caterpie's weight and falls on top of the city below. Suddenly, Caterpie gives off brilliant white light and evolves into Metapod, but remains perched atop the broken tower. Officer Jenny comes onto the scene and Dr. Gordon explains that this mess is his fault.
Jessie and James are upset over having lost the pills earlier, but Wobbuffet reveals that it grabbed three of them before they were blasted out of the lab. The trio recite their motto before seizing Dr. Gordon. Ash orders Pikachu to use Thunderbolt, but James sends out Cacnea to use Pin Missile. Pikachu manages to dodge until one of the pins scores a direct hit. Jessie sends out Dustox and throws the three pills towards it, but one falls short and lands in Cacnea's mouth. They both grow to about the same size as Metapod, to the surprise of everyone else. James tries to tell Cacnea a command, but it just turns around and falls onto James's relatively small body, crushing him into the ground. James returns Cacnea to its Poké Ball and the trio run away with the doctor in their possession and fly away on top of Dustox's back.
From atop the wrecked tower, Metapod's body begins to glow as it appears that it will evolve again soon. The Pokémon evolves into Butterfree, whose data May queries on her Pokédex. Flying through the sky, Dr. Gordon marvels at the experience of riding a Dustox. Ash and Xander appear on top of Butterfree, flying toward Dustox and Team Rocket. Jessie orders Dustox to use Poison Sting, which Butterfree dodges but the rest of the gang have trouble evading on the ground below as they ride along in Officer Jenny's off-road vehicle.
Pikachu launches a Thunderbolt at Dustox but misses. Ash notices steel electrical towers ahead of Dustox and directs Pikachu to use Thunderbolt on them, causing Dustox to narrowly avoid the power lines by diving recklessly. Xander tells Butterfree to use Supersonic, confusing Dustox and everyone aboard. The mysterious light appears around Dustox again as it quickly returns to normal size. This leaves Team Rocket and Dr. Gordon helplessly falling to the ground. Team Rocket falls into a lake below, but Butterfree saves the doctor from the fall.
Team Rocket starts wading to shore just as the rest of the group shows up with Officer Jenny. Jessie tells Dustox to use Psybeam on them, but Butterfree appears and Ash tells Pikachu to use Thunder on Team Rocket, who are still standing in the water. Xander tells Butterfree to use Whirlwind, blasting Team Rocket off with a gust of wind. Soon afterward, the mysterious light appears around Butterfree as it then returns to its normal size. Xander says that he's glad Butterfree is back to normal and is excited to go on a journey with it someday.