Which Wurmple's Which? (Japanese: ケムッソVSケムッソ！どっちがどっち！？ Wurmple VS Wurmple! Which is Which!?!) is the 21st episode of Pokémon: Advanced.
After being beaten by Brawly, Ash decides to do some serious training of one thing he lacks: patience. He does this by practicing the fine art of Fishing. He, along with May, Max and Brock, accidentally hook Team Rocket's Magikarp submarine and after a fierce battle, May notices something different about her Wurmple: it is actually Jessie's. Can the heroes find May's Wurmple before Team Rocket figures it out?
May wakes up, only to find Ash's bed empty. Ash's been waiting in Pokémon Center for healing of his Treecko and Taillow. Ash brings them out, looking better than ever. Ash apologizes to them for the battle they had. They have lunch and Ash decides to begin his training. Max comes to them, saying that Steven, son of President of Devon Corporation is doing some secret research, so they go to look for him.
While they walk, a man appears before them, saying that here many Water Pokémon live. Ash is convinced to do some fishing, but Team Rocket is near a bush while James is the man in disguise. Max gives May some advice, even if she says that she's old enough to know how to fish. May catches an Octillery, but she needs to battle it to catch it, so she sends Wurmple, but the Octillery goes on Max's head.
Wurmple uses String Shot on Max, even if he advises her to attack the Octillery. Octillery attacks Max and goes back into the water. Later, Ash catches a Corphish, even if it attacks Max first. The Corphish starts to attack everyone, so Ash's Pikachu uses Thunderbolt, but Corphish strikes first with Crabhammer. The Corphish goes to the water as well. The gang returns to fishing.
Team Rocket is frustrated by the fact that the gang are catching Pokémon. Jessie still continues to her plan, that they'll pull the twerps down and get their Poké Balls, along with Pikachu. May catches something, but it is really heavy, so everyone helps her catch it. As Team Rocket dives deeper, the gang puts more strength to get it out. Team Rocket has destroyed the breaks, so May gets the Magikarp submarine to the surface, revealing that Team Rocket is inside it.
Team Rocket steals Pikachu with an electric-proof net, so Treecko uses Quick Attack and Pikachu is free from the net. Meowth is angry while he threatens to steal everyone's Pokémon. May defends her Wurmple, but Jessie brings hers as well, arguing whose is better. Max breaks the discussion, beginning to analyze it. Max determines that both Wurmple are good, but this makes Jessie annoyed, so she orders her Wurmple to use String Shot. Upset, May tells her's to also use String Shot, but on Jessie. James is about to use the net, but Treecko spits the branch, making him drop the net. Treecko uses Quick Attack, making Meowth knocked out.
In doing so, he bounces to the two Wurmple, so May's and Jessie's get mixed up. Team Rocket attempts to escape, but Pikachu uses Thunderbolt, blasting them off. The gang is about to leave, but May offers Wurmple food, but Wurmple is not hungry, making her doubt it is hers. Brock says that it can be checked by putting the Pokémon back into the Poké Ball. However, when May does it, it does not get back, making the Wurmple Jessie's.
James and Meowth are about to eat cookies, but the Wurmple eats them all. James thinks that it might not be Jessie's, so when she wants a hug from it, the Wurmple resists. Jessie is not convinced either way, so Meowth asks it whose trainer does it belong to - it says it is hungry. Taillow has searched for Team Rocket, so May is sad when her Wurmple is in hands of Team Rocket.
Meanwhile, Jessie feeds the Wurmple, making an imagination how she caught it. Taillow has found Team Rocket, so the gang appears before them. May offers Jessie's Wurmple in exchange for her own. Jessie fools May, taking her Wurmple with her. They fly off in their balloon and Meowth makes an imagination how life would be when their boss, Giovanni, would receive an Wurmple - it would give him strings for his buttons. Taillow uses Wing Attack to cut the balloon, forcing May to get her Wurmple back.
Taillow uses Quick Attack, but Jessie's Seviper is sent out to use Haze, stopping Taillow. Treecko is sent out and it uses Pound, hitting Seviper. James sends his Cacnea to attack with Pin Missile. Ash sent out Pikachu, who avoids Pin Missile. Seviper uses Poison Tail, but Treecko dodges and uses Pound, hitting Seviper again and Pikachu uses Quick Attack, hitting Cacnea. As Team Rocket's Pokémon are sent flying to their trainers, Pikachu uses Thunderbolt to blast them off once and for all. May offers Wurmple some food, to which it eats quickly and sleeps, making it definitely her own.
- Featured Pokémon: Espeon
- This is the first time that the Team Rocket say "shipping" instead of "blasting" when they blast off in Hoenn.
- From this episode until Pasta La Vista!, Mike Pollock replaces Rodger Parsons as the Narrator. Pollock also narrates for episodes of Pokémon Chronicles.
- When Ash pulls Corphish out of the water Corphish has discoloring on the bottom of his claws.