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. Zander then appears and accuses the group of being thieves, so he has his Caterpie tie them up with a String Shot. The group protest, but Caterpie continues to encase them in String Shot. Dr. Gordon soon flies onto the scene, and uses his telepathic machine to predict their intentions. From the readings, Dr. Gordon predicts that Ash and his friends have a limousine waiting for them. Eventually, the group convinces Dr. Gordon and his student Zander that they are just travelers.
Zander apologizes for accosting them, before everyone introduces themselves back at Dr. Gordon’s laboratory. Ash compliment's Zander's Caterpie for its strong String Shot. Zander replies that Caterpie is his first Pokémon and he plans to go on a journey with it 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 device causes Dr. Gordon to fall fast asleep from the soothing melody.
Outside, Team Rocket spies on the lab with their binoculars. James admits the Doctor’s inventions could definitely help Team Rocket. Jessie remains concerned about Caterpie, given that it attacked them with String Shot the other night, though her teammates assure her that the invention blueprints will be worth the trouble. They enter the lab and begin their heist, but Caterpie notices the trio rummaging, followed by Dr. Gordon and the others. Caterpie heads into battle as Jessie calls upon her Seviper. When James picks up an orange case, Dr. Gordon becomes very concerned about the invention inside. 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. The Doctor yells as Caterpie then eats one. A reddish light appears around its body before Caterpie suddenly grows to enormous proportions, reaching the lab ceiling. Caterpie sends Team Rocket 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 increases in size yet, this time breaking through the ceiling. May tells Zander to return Caterpie to its Poké Ball, but he reveals that he 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 rushes onto the scene and Dr. Gordon explains that this mess is his fault.
Jessie and James are upset over having lost the mystery candy earlier, but Wobbuffet reveals that he grabbed three of them before they were blasted out of the lab. The trio spring into motion, reciting 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 candies 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 to hug its Trainer and falls onto James's relatively small body, crushing him into the ground. James returns Cacnea to its Poké Ball before the trio flees aboard Dustox with Dr. Gordon in their possession.
From atop the wrecked tower, Metapod's body begins to glow as it appears as it evolves into Butterfree. With their own winged gargantuan, Zander and Ash hop aboard Butterfree and give chase to Team Rocket. As the boys close in, 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. Zander 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 plummeting towards 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, who orders them under arrest. Jessie orders Dustox use Psybeam on them, but Butterfree appears and Ash has Pikachu use Thunder on Team Rocket, who are still standing in the water. Zander has Butterfree whip up a Whirlwind, blasting Team Rocket off. Soon afterward, the mysterious light appears around Butterfree as it returns to its normal size. Officer Jenny orders Dr. Gordon to lock the candies away, and he replies that they were an accidental invention. At that, Jenny suggests he keep the disturbances inside his lab. Zander says that he's glad Butterfree is back to normal and is excited to go on a journey with it someday.