Crossing a hill on the way to Goldenrod City, Ash and his friends notice a building nearby. As Brock is about to say what he thinks the building is, a Wooper drops from a tree and lands on Brock's neck, causing him to fall down the slope.
On the lower level, Olesia is about to teach the class of Wooper when Brock hits the ground. Wooper then jumps off Brock and Olesia asks if he is alright. Brock quickly recovers and starts flirting with her. Misty pulls him away and then asks Olesia about the buildings. Olesia explains that she loved to raise Wooper, so she went on to build the Wooper Pond preschool only for Wooper. Ash then consults his Pokédex for information on Wooper, which reminds him of his previous encounter with the Quagsire. Misty then asks Olesia if they could watch her teach and she agrees. She plays the tambourine and then the Wooper start to swim and dance. Ash and Misty then admire the cuteness of the Wooper while Brock admires Olesia.
In the woods, Jessie is looking around for food when she finds a mushroom. After James and Meowth tell her that it is poisonous, she tosses it aside and all three start to complain of how they haven't had any food for weeks. Then James says that they could call "the Boss" for money but Meowth reminds him that they have already spent all the money. They decide their only hope is to find a rare Pokémon. Wobbuffet pops out of his Poké Ball, but Jessie quickly recalls him.
Ash and his friends enjoy some tea and homemade biscuits with Olesia, but the picnic is interrupted when Olesia receives a call from her mom. Olesia is left concerned by the phone call and confirms with Ash's group that her mother has broken her leg after falling down some stairs. Brock then offers to take care of the Wooper while she is away. She accepts, gives Brock a book about raising Wooper, and drives off on her motorcycle.
Ash and his friends head inside the preschool's gates only to discover that the Wooper aren't in the pond. Pikachu then finds a trail of water that leads into one of the school's buildings. They follow the trail and find the Wooper eating and all over the place. Brock then reads in the book that the best way to get Wooper to listen is to show them affection and food. So Brock pulls out a piece of food and shows it to the Wooper. One Wooper walks up to Brock and proceeds to licking him, and soon the others follow suit. Ash then reads that the second best way was to play the tambourine. Misty then recalls that she used to play the tambourine when she was a little girl. She picks up the tambourine and begins to play it, but none of the Wooper seem to be entertained. Misty puts the instrument down and Togepi begins to play the tambourine. The Wooper are instantly swaying and follow Togepi to their outdoor pond. Misty is angry that they'd rather listen to Togepi's beat rather than her own. Brock says it's just because Pokémon know a Pokémon's beat. After all the Wooper have been returned to the pond, Brock congratulates Togepi on a job well done, and is relieved to have recovered all the Wooper. However, Misty realizes that one Wooper is missing; more specifically, Olesia's favorite one with a love heart marking on its chest.
Brock is shocked by the revelation and goes on to jingle the tambourine for the enjoyment of the Wooper. Misty and Ash leave Brock in charge of Togepi and the Wooper while they search for the missing one. As Ash and Misty look in one of the rooms a loud crash in the kitchen catches their attention. The crash came from the missing Wooper, however, just as Ash and Misty are about to retrieve it, it runs away, dodging all their attempts to catch it and almost destroying the room in the process; Ash and Misty hurry to stop a large pile of plates toppling over, and Pikachu is almost stabbed by falling cutlery, this results in the Wooper getting away. Outside, the Wooper is seen climbing the cliff face Brock fell down. Ash and Misty once again give chase. Wooper reaches the top of the cliff before Ash and Misty do, and runs off into the forest. It is later spotted crossing a very rickety rope bridge across a chasm. Ash runs across but is frozen with fear when hearing the creaking beams beneath his feet, Misty and Pikachu tentatively follow. Misty tries to reason with the Wooper to return to the pond, Wooper agrees, and begins jumping up and down in excitement. This makes the bridge begin to wobble, and once again the Wooper slips through their grasp and runs across to the other side of the bridge. The bridge then breaks leaving Ash and Misty hanging and Pikachu on the top having made it across. Ash tells Pikachu to leave them and chase the Wooper.
Meanwhile, Meowth is seen complaining over a boiling pot, claiming that mushroom soup won't have much flavor without any mushrooms. He then notices the Wooper, and, mistaking it for a mushroom, attempts to catch it. Jessie and James then return from their foraging, and, hearing about the "giant mushroom", start looking around for it, while the Wooper eats from their basket of mushrooms. They then spot the "giant mushroom" just as it eats the last of their mushrooms. The trio is at first angered by the Wooper, but its smiling face gets rid of this anger. Jessie decides it would be a good idea to take it for their boss, thinking that Wooper's overwhelming cuteness may cause Giovanni to forget about their many failures. Jessie throws out a Poke Ball, but it fails as Wooper already has a Trainer. Pikachu then arrives to Wooper's defense, and refuses Team Rocket's request to convince the Wooper to enter the Poké Ball for them.
Ash and Misty rush onto the scene. Team Rocket reveal their nets and soon everyone is scrambling to grab Wooper, but its slippery skin is making it a difficult task. Jessie then commands Wobbuffet to attack the twerps, but he does nothing. She then sends Arbok clashing into the group, and James orders his Weezing to use Smokescreen. Team Rocket escapes in their balloon with Wooper netted, but at that moment Olesia and Brock arrive on a motorbike to give chase. Team Rocket then recites their motto. Before Brock and the others release their Pokémon to free the Wooper, Olesia thinks she can entice it to return by telling it that playtime is over and it has to go back inside. Wooper promptly jumps out of the net and back to the group as Arbok and Weezing pursue it. Wooper looks in trouble, but Olesia commands it to use Tackle attack on both of them, revealing it to be a strong Wooper. On command, Wooper tosses Arbok into Weezing, and the hurtling Pokémon collide with the rest of Team Rocket before being blasting off.
Back at the school, Togepi had been keeping the group entertained with its tambourine. Brock asks what Olesia's secret is for raising such strong Pokémon, to which she replies that there isn't one, and the strength really comes from people not expecting the Wooper to be powerful. Brock then asks if he can stay at the school so they can both learn about Pokémon and "maybe each other", resulting in him getting dragged away by Misty. The group then says their goodbyes, and continues on their way through Johto.