While traveling through the woods, Ash, Misty and Brock find an Eevee that was apparently abandoned next to a tree. Ash wants to keep it, but Misty points out the collar its wearing. Ash checks the Pokédex for information about Eevee and it says that Eevee can evolve into three different forms using three kinds of stones. When Misty learns of Eevee's stone evolutions, she wants to keep it and evolve it into a Vaporeon. They take the Eevee to the adreess written on its collar in Stone Town.
Ash and his friends take the Eevee to the house written on the collar and they find out that its the biggest house in town.They look into the backyard and they see a big evolution party going on. There are many trainers there evolving their Pokémon with evolution stones.
Team Rocket looks down at the party from above and they see all of the evolved Pokémon. James admires all of the Pokémon while Meowth reads from a book that says Stone Town is at the bottom of Evolution Mountain, a mountain that has many evolution stones in it. They make a plan to steal all of the Pokémon, the stones, and the food at the party.
The three brothers who are hosting the party (Rainer, Sparky, and Pyro) are happy to see Ash and his friends with Eevee. They call over their little brother, Mikey, who is the owner of Eevee. Rainer, Sparky, and Pyro tell Mikey that he must evolve Eevee or it will never win in a battle. Mikey doesn't want to evolve Eevee, but his brothers won't listen. Each of Mikey's brothers tries to convince him to evolve Eevee, but Mikey refuses. Ash tells them to leave MIkey alone, but Rainer, Sparky, and Pyro start trying to force Ash and Brock to evolve Pikachu and Vulpix. They offer Fire and Thunder Stones to Ash and Brock and they ask them if they would join the Evolution Club by evolving Pikachu and Vulpix. Ash and Brock both refuse.
Misty lets Horsea out of its Poké Ball so it can swim in a fountain. Misty sees Mikey siting in a corner by himself with Eevee. Misty takes a tray of food over to Mikey and asks him if he would like some food. Mikey takes a sandwich and Eevee takes some Pokémon food. Misty says she noticed that he isn't too excited to evolve Eevee. Mikey tells her that he tried to hide Eevee in the forest for the day so Rainer, Sparky, and Pyro wouldn't make him evolve Eevee. Mikey says he doesn't want to evolve Eevee, he just wants to be Eevee's friend. Misty surprises Mikey by saying she agrees with him. Misty tells him the best thing he should do is to tell his brothers how he feels.
Just as Mikey is about to tell his brothers how he feels about Eevee, Team Rocket crashes the party. James tells Weezing to use Smokescreen. When Ash has Pidgeotto use Gust to blow away the smoke, Team Rocket has disappeared with all of the Pokémon, the evolution stones, and the food. Team Rocket leaves their balloon at the party and they make a getaway in a jeep. But Horsea leaves a trail of Sludge for Ash and his friends to follow.
After they eat all of the food, Team Rocket decides to evolve Eevee but they can't decide what to evolve it into. They decide to use a Water Stone, a Fire Stone, and a Thunderstone all at once on Eevee and see what happens. They are about to when Squirtle stops them. Team Rocket asks Ash and his friends how they found them and Misty tells them about the trail of sludge Horsea left. Ash and his friends, Mikey, and his brothers start to battle Team Rocket.
During the fight, Mikey's brothers' Pokémon lose and Mikey steps up to take their place. Mikey proves his Eevee can fight well just as it is. Eevee and Pikachu send Team Rocket blasting off again. Mikey tells his brothers he wants to keep his Eevee just how it is and come an Eevee trainer. They agree after seeing how strong Eevee was and they let Mikey keep his Eevee the way it is.
- Eevee (Mikey's)
- Flareon (Pyro's)
- Vaporeon (Rainer's)
- Jolteon (Sparky's)
- Misty's Psyduck is revealed to know water gun.
- When Misty is talking to Mikey, Pikachu's left ear is missing.
- Misty's Horsea uses Sludge on Jessie, even though a Horsea can't learn Sludge in the games.
- Pyro, Rainer and Sparky's names are connected to the types that their Pokémon are (Fire, Water and Electric)
- This is the first episode to introduce evolution stones; fire stones, water stones, thunder stones and leaf stones.
- Ash and Brock are asked to become members of the Evolution Club if they evolve their Pokémon using the evolution stones; Brock with his Vulpix and Ash with his Pikachu.
- This is the second time that Ash is persuaded to evolve his Pikachu with a Thunder Stone, the first being Episode 14.
- This episode features another recurring gag: Calling Jessie an Old Lady. Another character voiced by Rachael Lillis (Ursula from Dinosaur King), also suffers from this gag.
- The Eevee Brothers hair colours and styles match their version of Eevee, Rainers hair is blue for Vaporeon, Pyro's hair matches Flareon while Sparky's hair is yellow for Jolteon. Mikey's hair is brown like Eevee.
- The "Who's that Pokémon?" in this episode is Eevee.
- When Team Rocket crashes the party and starts eating the food, you see James eating a lobster with what looks like a bite mark on its shell. This might mean that James might have taken a direct bite out of the lobsters shell which is hard and not very tasty.
- Misty: "I don't think you should."