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?
At the Pokémon Center, as Brock and Max are sleeping May wakes up, only to find Ash's bed empty. Max and Brock also wake up and see May is correct. Ash's been waiting in lobby, to be close to Treecko and Taillow, who are being healed. Nurse Joy comes out and reports Treecko will be fine. After some time, Nurse Joy gives Ash back his Poké Balls and brings out Treecko and Taillow. Ash apologizes to them for the battle they had, who forgive him. They have breakfast and Ash promises to begin his training, while Brock recalls anywhere on the island is fine for Ash to train. Max comes to them, hearing Steven, son of Mr. Stone, president of Devon Corporation, is doing some secret research, so they promise to look for him. Max recalls he is in Granite Cave, looking for rare stones.
As the heroes walk, a man appears before them, saying that here many Water Pokémon live. As Ash is convinced to do some fishing, Jessie and Meowth are near a bush while James is the man in disguise, pleased that their foes fell for the bait. As the heroes are fishing, Ash sends Treecko to enjoy the sunshine and nature. Max gives May some advice, even if she says that she's old enough to know how to fish. May fishes out an Octillery, but since she needs to battle it to catch it, so she sends Wurmple. However, Octillery goes on Max's head, using Constrict on him. Wurmple uses String Shot, but Octillery jumps away, causing String Shot to land on Max. Octillery Headbutts Max, then goes back into the water. Later, Ash fishes out a Corphish, even if it attacks Max first, by pinching him with Vice Grip. The Corphish starts running around, causing everyone to be startled. Ash's Pikachu goes to use Thunderbolt, but Corphish strikes first with Crabhammer, then dives back into the water. The heroes decide to return to fishing.
Team Rocket is frustrated by the fact that their foes are successfully fishing out Pokémon. Jessie still continues to her plan; they'll pull the others 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 heroes puts more strength to get it out. Suddenly, Team Rocket has destroyed the breaks, so May fishes out the Magikarp submarine to the surface, revealing Team Rocket inside it. Team Rocket steals Pikachu with an electric-proof net. Treecko uses Quick Attack, knocking James away and freeing Pikachu from the net. Meowth is angry while he threatens to steal everyone's Pokémon. May defends her Wurmple, but Jessie sends hers as well, arguing whose is better. Max breaks the dispute, beginning to measure Wurmple. Max determines that both Wurmple are equally same, but this annoys Jessie, who has her Wurmple use String Shot. Upset about this, May has her Wurmple use the same attack on Jessie. James is about to throw the net, but Treecko spits the twig on his hand, causing him to drop the net. Treecko uses Quick Attack, knocking Meowth out.
In doing so, Meowth bounces onto the Wurmple, so May and Jessie's Wurmple get mixed up. Team Rocket attempts to escape, but Pikachu uses Thunderbolt, blasting them off. The heroes are about to leave, but May suspects something about Wurmple and offers it food. Wurmple is not hungry, making May doubt this Wurmple is her own. Brock says that it can be checked by putting it back into the Poké Ball. However, when May does it, it does not return to its Ball, making the Wurmple as Jessie's. The heroes are worried, but May promises to return her Wurmple back. James and Meowth are about to eat cookies, but the Wurmple eats all the food. Meowth and James are startled, but Jessie claims Wurmple just has an appetite. James thinks that it might not be Jessie's, so when Jessie wants a hug from it, the Wurmple refuses. Jessie is not convinced either way, so Meowth asks Wurmple whose trainer does it belong to, but it says it is still hungry.
Taillow has searched for Team Rocket, but no luck. May is sad when her Wurmple is in hands of Team Rocket. May remembers she was startled by the Wurmple, but it grew on her and, with Torchic's help, May caught the Wurmple. This makes May nearly cry, as she wonders what she'll do if she does not return her Wurmple back. Meanwhile, Jessie hugs Wurmple and has a fantasy on how she caught it. Taillow has found Team Rocket, so the heroes appears before them. May claims she is here to switch Jessie's Wurmple in exchange for her own. Jessie and May step away and both offer their Wurmple to exchange for the other. However, Jessie fools May, taking both Wurmple with her. Team Rocket flies off in their balloon and Meowth imagines how life would be when their boss would love when Wurmple would offer strings for his buttons. Taillow uses Wing Attack to cut the balloon, causing it to fall down.
May comes to the balloon and takes her Wurmple back, while Jessie sends Seviper to steal back May's Wurmple. Taillow uses Quick Attack, but Seviper uses Haze, stopping Taillow. Treecko is sent out and uses Pound, hitting Seviper. James sends his Cacnea to attack with Pin Missile. Ash sent out Pikachu, who avoids Cacnea's Pin Missile. Seviper uses Poison Tail, but Treecko dodges and uses Pound, hitting Seviper again. Pikachu uses Quick Attack, attacking Cacnea. As Team Rocket's Pokémon are sent flying to their Trainers, Pikachu uses Thunderbolt to blast them off once again. Later on, May gives Wurmple some food, who eats quickly and sleeps. Ash wonders how May knew from the start that was not her Wurmple. May admits she sensed it, while Brock reminds them it is due to the bond with the Pokémon Trainers can sense such stuff. Max sees May is becoming a better Trainer, while May thinks it is due to Wurmple and thanks it.
- Featured Pokémon: Espeon
- From this episode until Pasta La Vista!, Mike Pollock replaces Rodger Parsons as the Narrator. Pollock also narrates for episodes of Pokémon Chronicles.
After Ash pulls Corphish out of the water, Corphish has discoloring on the bottom of his claws.