On their way to Cinnabar Island for the seventh Gym battle, Ash and his friends are walking through a forest when Pikachu hears a Pokémon's cry. It leads the group towards the cry and shows them that inside of the base of a tree is an Eevee tied to a post. After scanning Eevee with his Pokédex, Ash wants to catch it since it may be abandoned, but Misty notices Eevee has a name tag and address on it saying that its trainer lives in Stone Town. Brock says to them that Stone Town is at the foot of Evolution Mountain, which contains evolution stones that allow certain Pokémon to evolve. On their way to Stone Town, Ash looks in his Pokédex for more information on Eevee, and it mentions Eeeve's evolutions via evolution stones: Jolteon, Flareon, and Vaporeon. The group finds that the address leads them to giant mansion.
As they arrive in the backyard, the heroes notice that there is a garden party being held. They see that almost all of the Pokémon at the party are ones that evolve with evolution stones. They even get the chance to see many of the said stones out on display. Meanwhile, up on top of a hill, unbeknownst to everyone, Team Rocket sees all of the evolved Pokémon and hear about the stones, and also crave for the food.
The three brothers who are hosting the party, Sparky, Rainer, and Pyro, see Ash and his friends with Eevee, and says that that Eevee is the star of the party. They call for their youngest brother, Mikey, who is Eevee's trainer. The brothers, who each own one of the Eevee stone evolutions, mention that the only way to win battles is for Mikey to evolve his Pokémon. Mikey says that he doesn't care about battles, which makes his brothers and their Pokémon mad, and they push Mikey even harder about which of the Eevee evolutions is stronger. Brock comments that the family sure is competitive, especially when they try to push Ash to evolve Pikachu and Brock to evolve Vulpix.
As Misty watches her Horsea swim around in the fountain, she looks over to see Mikey on the bench with a drink, and offers some food to him, telling him not to let his brothers push him around. Misty asks if he really wants to evolve Eevee, and Mikey explains that his brothers only care about battling, so he hid his Eevee just for today so that nobody could make him evolve it. Misty tells him that he was just running away from his problems, and that it is his responsibility as a Pokémon Trainer to tell his brothers that he doesn't want Eevee to evolve. Pikachu plays with Eevee, and Misty mentions how it once had the chance to evolve and chose not to.
Mikey's brothers come up and ask Mikey if he has made his decision yet. Misty tells Mikey just to go right out and tell his brothers what he has wanted to say. As Mikey is about to tell his brothers what he has decided, sparks on the ground go off by everyone. Team Rocket arrives in their balloon and does their motto while eating all of the party food. Weezing then uses SmokeScreen over everyone. Ash brings out Pidgeotto to use Gust to blow the SmokeScreen away. Once the smoke clears, everyone realizes that all the Pokémon (except Misty's Psyduck, much to her annoyance), the evolution stones, and the food are all missing.
Brock notices that they are getting away in their balloon, and Ash has Pidgeotto stop it. The balloon crashes, but it turns out to be a decoy when it turns up empty. Pidgeotto goes to search for Team Rocket. As Team Rocket are walking down the road with the Pokémon in cages, Horsea leaves a trail of ink so that everyone will know where they are headed to. Pidgeotto sees the ink on the ground.
At the lake with the trapped Pokémon, Team Rocket has a picnic with the food that they stole. They realize that they need to decide what to evolve Eevee into. They soon get to arguing about it, until Jessie blows a whistle and says that they should use all evolution stones at once. They go to evolve Eevee, when Ash has Squirtle use Water Gun to stop them. Before it can attack, Psyduck appears in front of Squirtle and tries a Water Gun but fails. Team Rocket says that is embarrassing, as Misty agrees and returns Psyduck to its Poké Ball.
Squirtle uses Water Gun on James and Meowth, which sends them backwards, and Jessie tells them to stop as she picks up Eevee's cage. She brings out Arbok, while James brings out Weezing to attack Misty. Misty and Ash prepare to battle with their Pokémon, but the Eevee Brothers stop them and say that they must be the ones to battle, since it is their brother's Eevee that was stolen.
The brothers get ready to attack with Vaporeon, Jolteon, and Flareon. Arbok uses Poison Sting, while Jolteon counters with Pin Missile which hits Arbok. Weezing uses Sludge, but Vaporeon enters the water and uses Acid Armor to go invisible to avoid the attack. Weezing's attack misses as Vaporeon jumps out of the water and uses Tackle to send Weezing to the ground on top of James. Team Rocket tries to run away, but Flareon uses Fire Spin to stop them. The brothers demand that Team Rocket give back the stolen Pokémon, but Jessie has Arbok use Tackle on Jolteon, while Weezing uses Sludge on Flareon and Vaporeon. Horsea spits out ink into Jessie's face which sends Eevee flying and it lands in Mikey's arms. Pikachu uses Thunderbolt on Team Rocket, but they keep going.
As Mikey pulls Eevee out of the cage, he looks over and notices that his brothers' Pokémon are severely hurt. Team Rocket decides that it's time to finish them off. Mikey stands up and decides that he and Eevee are going to have their first battle. Mikey commands Eevee to use Take Down, and then has Eevee turn it into a rage Tackle. Eevee sends Team Rocket blasting off with the attack. Mikey jumps for joy and picks up Eevee after winning, while Eevee licks him to show affection. Mikey's brothers congratulate him on his first win, and Mikey tells them that he just wanted to help everybody out. He also tells his brothers that he wants to become an Eevee Trainer. The Eevee brother accept his decision, and Sparky says that they should turn the party into a celebration of Mikey's first victory.
As they do cheers with drink glasses, everyone congratulates Mikey on his first battle victory. Mikey thanks Misty for helping him, gives her a flower, and they shake pinky fingers and promise to be friends forever. Everyone at the party then gathers for a picture. Psyduck clicks a button to take the picture, but falls as the camera takes the shot.
Meanwhile, later in the night, Team Rocket is eating food that isn't as fancy. Jessie says that they will be champions of the world, as they all say "some day".