Ash and Co. are traveling on a forest path when Pikachu notices an Eevee chained to a nearby tree. Brock thinks it's abandoned, and Ash and Misty immediately want to take it with them. Misty then notices a nametag on the collar of the Eevee. It reads "Stone Town, House 3-14." So Ash and Co. set out to return the Eevee to its owner.
Stone Town is at the base of Evolution Mountain, where all the evolutionary stones can be found. House 3-14 turns out to be a very large, very nice mansion. There's a backyard party going on, and Ash and Co. drop in uninvited. Three brothers, are throwing the party for their younger brother, Mike, who is to decide what he will evolve his Eevee into. It turns out that the Eevee belongs to this boy. The reason, Misty finds out, that the boy chained up Eevee in the woods was so he wouldn't have to evolve it today, as he likes Eevee just the way it is. But his three brothers, who have Vaporieon, Flareon, and Jolteon, are overanxious to evolve the Eevee and seem to be overpowering their younger brother. These brothers seem to think that the only way to win battles is to have an evolved Pokemon. Ash sticks up for Mike, saying you don't need an evolved Pokemon to win, and that leads to an argument between the brothers and Ash and Brock over when to evolve Pokemon, as early as possible or when the Pokemon are ready. The brothers end up offering to evolve Pikachu and Vulpix, but Ash and Brock decline. While this is happening, Misty is counseling Mike on standing up to his brothers and refusing to evolve Eevee, and not running away from his problems by hiding Eevee. Ash and Brock are interested in other things, as they are checking out all the rare Pokemon, all the evolutionary stones, and all the food. However, the same things that are attracting Ash and Brock's attention have also attracted Team Rocket to the party. Just as Mike is about to announce he doesn't want Eevee to evolve, TR descends to the lawn in their balloon. They're so hungry they start eating food while they recite their motto. But they soon get down to business, as Wheezing spreads a smoke screen and TR takes all the evolutionary stones and Pokemon. When the smoke clears, Ash sees the Meowth balloon getting away, and sends up Pidgeotto to pop it. However, it turns out the balloon was a decoy; there was no one in it. TR stole a truck and escaped by way of a back road with a good deal of Pokemon, including Horsea, Vulpix, and Pikachu, and all the Eevees. However, Horsea squirts out ink to mark a trail on the road as TR flees, and Pidgeotto spots it…
We join TR by a lake, chowing down on all the food they stole. They decide to try and evolve the Eevee, but can't decide which version to evolve it to. Finally, they decide to use all the stones at once, but before they can get the first Vaporjoltflareon, Ash and Co. shows up and Squirtle uses Water Gun to blast the stones away. They had followed the trail of ink left by Horsea that Pidgeotto found to the TR campsite. After freeing most of the Pokemon, Ash and Co. discover TR still has Eevee, and the Three Eevee brothers use their Eevee evolutions to battle TR. However, TR battles back and actually beats up the Eevee trainers, but Eevee escapes from his cage and returns to Mike. After Pikachu shocks TR real good, Eevee uses Rage Tackle and blasts off TR.
After the battle, Mike tells his brothers that he wants to keep his Eevee as it is. His brothers, after watching Eevee beat TR, agree that Mike should be able to choose not to evolve Eevee. They welcome his decision to keep Eevee, and modify the party to celebrate Mike and Eevee's first battle victory. And while Ash and Co. enjoys the good food at the party, TR is eating food straight from the tin cans in the wilderness. Oh well, maybe next time.