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.
Ash and his friends wake up in the Pokémon Center and promptly notice that Ash and Pikachu are missing. In the hallway outside, they find the two sleeping on a bench, just to be closer to Ash's injured Treecko and Taillow. After receiving his fully healed Pokémon from Nurse Joy, the gang eat breakfast and discuss Ash's training. Max approaches the group and says that Steven Stone, the son of Mr. Stone, is in Granite Cave doing some secret research. Ash and his friends decide to meet him there. 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 rods. As the gang starts fishing, Ash lets Treecko out to enjoy the sunshine.
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 uses Constrict on Max. May sends out her Wurmple and commands it to use String Shot, but Octillery dodges. Wurmple mistakenly ensnares Max with the attack. Octillery escapes back into the sea, taking the time to Headbutt 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 explains their plan to steal Pikachu and the other Pokémon. Team Rocket intends to snare the gang's rods and pull them into the water, then steal their floating Poké Balls, along with an exhausted Pikachu. Fortunately, their pedals break and Ash and friends pull them to the surface. Team Rocket exits their submarine while saying their motto, then try to capture Pikachu in a shock-proof net. Ash commands Treecko to Quick Attack James, who drops the net. Meowth, claws extended, threatens to steal all of the gang's Pokémon. May quickly defends her Wurmple, beginning an argument with Jessie over whose Wurmple is superior. Max tries to settle the fight by declaring both Wurmple equal, but Jessie is determined to battle. She commands her Wurmple to use String Shot, but misses. May and Ash command Treecko and Wurmple to fight, and the two Wurmple are swapped during the confusion. Team Rocket tries to flee, but are blasted away by Pikachu's Thunderbolt.
Afterwards, May notices that the Wurmple isn't hungry, and 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 gang 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 the gang. 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 jump in their Meowth balloon while fantasizing about giving the Wurmple to their boss. Taillow interrupts their 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. The episode ends with the gang reflecting on the bond between Pokémon and Trainer, happily looking forward to future training.