Which Wurmple's Which? (Japanese: ケムッソVSケムッソ！どっちがどっち！？Kemusso VS Kemusso! Which is Which!?) is the 21st episode of the Advanced Generation series, and the 295th episode of the Pokémon anime. It was first broadcast in Japan on April 17, 2003 and in the United States on February 14, 2004.
Everyone is getting up and getting ready for breakfast, but Ash is nowhere in sight. He is found outside the Pokémon Center, anxiously awaiting news on his Treecko. After breakfast, Ash and company head out to Granite Cave because they heard one of the elite Trainers is there. On their way, they pass an old man who says that this is the ultimate fishing spot where rare and unusual water Pokémon can be caught. The man then offers them some fishing poles to find out for themselves. What our heroes aren't aware of is that the old man is actually James and the other two Team Rocket hooligans are hiding in the bushes. After some time, May catches an Octillery. She tries to battle it with her Wurmple, but fails and the Octillery jumps back into the water. Ash has the same luck with the Corphish he caught.
Throughout all this, Team Rocket is hiding in the water in their Magikarp sub, waiting for someone to snag their sub so that they can pull everyone, Poké Balls and all, into the water. May is the unlucky candidate, but instead of Team Rocket pulling her down, Ash, Max, Brock, and May pull them up and out of the water. After reciting their motto, Team Rocket jumps onto the shore and tries to steal all the Pokémon. When May defends her Wurmple, Jessie says she doesn't want May's Wurmple because Jessie's is better. A fight between the girls ensues over which one is better. Max concludes that the Wurmples are about the same. James tries to steal the Pokémon again and in the resulting confusion, the Wurmples get mixed up. Later, May realizes what happened when her Wurmple won't go back into its ball. Ash's Taillow helps the gang find Team Rocket to make the trade. When May and Jessie move to trade back, Jessie takes both Wurmples and tries to escape in Team Rocket's balloon. Ash sends Taillow to pop the balloon and a Pokémon battle results. The episode ends with May happily feeding her Wurmple.
On the way to Granite Cave, they come across James, disguised as an old man. He convinces them to fish at a nearby fishing spot, and lends them some rods. Ash is set on getting some training in, but Brock reminds him of Brawly’s advice; even play can be training. As the group starts fishing, Ash lets Treecko out to enjoy the sunshine. Max tries to instruct his older sister how to fish, but May retorts that she is the older one. After fishing for a moment, May lands an Octillery. She looks it up in her Pokédex as Brock explains that she needs to battle the hooked Pokémon. Behind them, the wild Octillery suctions itself onto Max’s head. May sends out her Wurmple and commands it to use String Shot, but Octillery dodges. Wurmple mistakenly ensnares Max with the attack instead. Octillery escapes back into the sea, taking the time to headbutts Max on the way. Next, Ash lands a Corphish, which grabs Max using Vice Grip. Pikachu tries to use a Thunderbolt, but Corphish quickly attacks with Crabhammer before returning to the water.
Meanwhile, Team Rocket is back inside their Magikarp submarine. Jessie isn't too thrilled that the random fishing spot is helping the twerps land several catches. She goes onto explain the next plan of attack: Team Rocket intends to snare the group's rods and pull them into the water, then steal their floating Poké Balls, along with an exhausted Pikachu. May soon feels something on her line, and the other lend their strengths. The combined effort causes Team Rocket's pedals to break allowing the group to pull the sub to the surface. Team Rocket exits their submarine while reciting their motto, before trying to capture Pikachu in a shock-proof net. Ash commands Treecko to use Quick Attack James, who drops the net. Meowth, claws extended, threatens to steal all of the twerps' Pokémon. May quickly defends her Wurmple, beginning an argument with Jessie over whose Wurmple is superior. Max tries to settle the fight, so he measures and closely examines the two Wurmple and comes to the conclusion that they are both equal. Jessie doesn't agree with the verdict and decides a battle is in order. She commands her Wurmple to use String Shot, but it misses. May has her Wurmple use String Shot, and the silky threads hit Jessie. James tries to the cast the net once again, but Treecko spits its twig at him. May and Ash command Treecko and Wurmple to fight, and the two Wurmple are swapped during the confusion. Jessie carries the injured Wurmple and Meowth under her arms as Team Rocket tries to flee, but they are blasted away by Pikachu's Thunderbolt.
Afterwards, May notices that the Wurmple isn't hungry, and is beginning to sense something is wrong. She follows Brock's suggestion and tries to recall Wurmple back to its Poké Ball, but it doesn't return as expected. The group figures out that the two Worm Pokémon must have been switched. Meanwhile, Team Rocket is dismayed to find that the Wurmple near them has a ferocious appetite, and doesn't seem to like Jessie. The group hunts for Team Rocket, using Taillow to scout the area. Both May and Jessie reminisce about capturing their respective Wurmple, but Jessie's reminiscence is cut short by the arrival of Ash and his friends. May reveals the truth about the swap, and Jessie agrees to give both Wurmple to their correct Trainer. However, Jessie double-crosses May, stealing both Wurmple and running away. Team Rocket jumps in their Meowth balloon while fantasizing about giving the Wurmple to their boss. Taillow interrupts their boss fantasy, and deflates their balloon with a quick Wing Attack. May recovers her beloved Wurmple, but Jessie sends out Seviper to battle.
Taillow tries to use Quick Attack, but Seviper's Haze quickly stops it. Ash sends out Treecko to use Pound on Seviper and James counters by sending out his Cacnea, but it hugs him instead. Ash sends Pikachu next, but Team Rocket isn't finished yet. Cacnea uses Pin Missile while Seviper uses Poison Tail. Treecko and Pikachu dodge the attacks and they use Pound and Quick Attack to finish off Seviper and Cacnea. Pikachu sends Team Rocket flying with a final Thunderbolt. While James and the others aren't thrilled about another blast off, though Jessie is just glad to be reunited with her darling Wurmple again. May is equally as happy, and offers Wurmple a bowl of food, which it quickly polishes off before falling asleep. Max and the other remark that May is becoming quite the Trainer. May replies that her growing confidence is all thanks to Wurmple.