Ash and his friends are traveling through a forest with many Pokémon. Max keeps exclaiming loudly and moving towards various Pokémon, scaring them away. Then a group of Mightyena come around and display hostility. Ash's Pikachu, Brock's Forretress and May's Torchic are ordered to attack them. They start to battle, but then a Poochyena comes running in, followed by Katrina. She explains that this is a Pokémon reserve. Brock starts trying to woo her, causing her to get uncomfortable, and the Mightyena attack him. The scene cuts to Team Rocket, who have heard that the woods are supposed to be full of Pokémon, but haven't seen any. Then they notice one and suddenly they spot several more that they'd overlooked. Wobbuffet pops up, holding some kind of sign. Reading it, they see that it's a notice indicating that this is a Pokémon reserve. They immediately start plotting to steal Pokémon.
It cuts back to Ash and his friends, who have followed Katrina to the preserve center, when the sick and injured Pokémon are cared for. Brock starts claiming that he, too, is a doctor and while going on about it, the Poochyena Tackles him. Katrina scolds it and Brock claims that she even scolds beautifully! Soon after, Poochyena is sleeping and Max listens to the Pokédex entry about it. Then he and Katrina start talking about when Poochyena will evolve, and she reveals that Poochyena is the same age as the group of Mightyena they met, but hasn't evolved yet, probably because it only uses Tackle. Katrina says that she is not worried - it's not a problem if it takes a long time to evolve or even if it never evolves. Max protests that Mightyena are much more powerful than Poochyena, and starts extolling the benefits of evolving to the Poochyena.
May starts thinking about her Torchic evolving, and her and Max talk about how much he wants to see a Pokémon evolve and how they might get her Torchic to evolve. Ash tells them they can't just get a Pokémon to evolve because they want it to - they have to gain experience first, and it takes a while. Max complains about why things take so long, and Brock says if it were easy to evolve Pokémon, it would not be so fun. Ash elaborates that Pokémon only evolve after they've done a lot of work with their Trainer, and says that the getting to know each other as the Pokémon are getting experience is part of the experience. They start talking about why Pikachu has not evolved after all this time, and Ash says there are different kinds of evolution. Brock explains that some Pokémon, such as Torchic and Poochyena, evolve with experience, while others such as Pikachu need a certain item to evolve.
Max insists he wants to stay there awhile because Poochyena must be ready to evolve any time. Katrina says there's no guarantee Poochyena will evolve soon, but she does have room if they want to stay. Ash wants to hurry on his way to challenge the next Gym, but Brock immediately accepts. Max tells Poochyena he'll do everything he can to help it evolve. It cuts to Max getting Poochyena to battle Torchic and urging him to use Bite instead of Tackle. Torchic finally gets fed up and scorches both of them before walking away. Then it cuts to Ash feeding Pikachu a snack and Max comes in to ask if he can borrow Pikachu or another of Ash's Pokémon to fight Poochyena. May says he'll probably just get them to lose deliberately so Poochyena can evolve, to which Ash says that such faked battles aren't good for Poochyena.
Later, Max is watching Poochyena fight one of the Mightyena, using Tackle over and over, and get discouraged. Ash comes up and says lots of Pokémon are happy the way they are and don't want to evolve, such as Pikachu. He asks if Max understands, and Max says he knows Ash doesn't want him to cheat, and unexpectedly calls Poochyena and runs off with him. Ash and May are puzzled, but then Katrina comes up and shows them part of a trap for catching Pokémon. Ash and May realize there are poachers in the forest. It cuts to Team Rocket standing beside a pile of Pokémon trapped in a net and saying "we rock!" Then they start fantasizing about a promotion and the benefits that will result, such as Jessie getting a new house.
It cuts to Max and Poochyena, Max saying "this looks like a good spot" and discussing plans to attack any Pokémon who aren't flying-type and don't have special attacks. Team Rocket is watching and plots to steal the Poochyena from the "mini-twerp". Max then tells Poochyena that although it is good at tackling, it's only effective if the opponent is its size or smaller, so he has to start using Bite. Team Rockets comments that he's trying to evolve it into a Mightyena, and decide to give him some "help". Max then starts looking for something for Poochyena to attack, and spots a machine. He starts examining it with interest, then decides to just walk away from it. Team Rocket, disguised as scientists, come up and explain that they're doing top secret research and don't want Max to tell anyone about it. Max agrees, then asks what they're researching. James says they don't want to tell him, but "unintentionally" let it slip in the process. He gets sheepish, and Jessie says that now that Max knows, why shouldn't they demonstrate? They put Meowth into the machine, pull a lever, and open it to reveal Meowth in a Persian costume. Max is fooled and very amazed.
Max asks if they can do it some more, and they turn the "Persian" into a Sunflora, which really surprises Max, who protests that it's not possible for a Persian to evolve into a Sunflora. Jessie says that's what they told Albert Einstein before he invented the light bulb, to which Max says Thomas Edison invented the light bulb. They quickly move on and proceed to go through a series of other such changes, then it cuts to Meowth trying to decide which costume to put on next. He delays too long and they knock on the machine, so he hurriedly puts on several pieces of different costumes. This really makes Max suspicious. Jessie and James stand in front of the machine and offer to give the Poochyena a try. Max says he knows it's rigged and starts to leave with Poochyena, but Jessie snatches it and reassures him that they do it all the time. They put Poochyena into the machine, and a moment later are holding Max back as he tries to get free. They tell Meowth to go ahead and the machine appears to explode.
Next thing, Team Rocket is thanking Max as they are about to leave with Poochyena and the other stolen Pokémon in their Meowth balloon. Max insists they give it back, and they refuse. Just then, Katrina, Ash and friends arrive, and Katrina accuses Team Rocket of being the poachers and they proceed to unmask themselves and do the motto. Then they use a machine to grab Pikachu and start to float away. Katrina, Ash and his friends give chase. Ash releases his Taillow, who starts Pecking Jessie and James. They stop piloting their balloon and it gradually floats to the ground, for which Ash congratulates Taillow. Max urges Poochyena to bite through the net in order to free himself and all his friends. Poochyena is scared, but with enough encouragement, he does it. They all fall to the ground, Team Rocket looks dismayed, and just then Taillow's persistent pecking causes James to loosen his hold on the Pikachu-grabber, and it releases Pikachu. Then Ash orders Pikachu to use Thunderbolt, which causes Team Rocket to blast off again.
It cuts to Max congratulating Poochyena for finally using Bite and saving the day. He says Ash is right - Poochyena's great whether he evolves or not. Poochyena comments happily and then begins to evolve! Max and May don't understand why Poochyena is glowing, but Katrina explains that it's beginning to evolve. The newly evolved Mightyena lets out a triumphant howl, Max is amazed, and the other three Mightyena come to congratulate their companion.