Ash and his friends are resting in an area where Farfetch'd, a rare Pokémon has been sighted. When Ash and Brock leave to get some water, Misty spots a Farfetch'd. It runs away, and she follows. After a while, she crashes into a boy, dropping her bag. The boy hands her back her bag and quickly departs.
When Misty returns to Ash and Brock, she discovers that she has the wrong bag: It's filled with rocks instead of Pokéballs. The boy, who owns the Farfetch'd, used his Pokémon to steal her bag. And when he spots Team Rocket, he decides to try his tricks on them as well. He convinces them that he wants to get rid of his Farfetch'd. When he leaves to "get something from his tent" he leaves his bag (which he says contains Pokéballs) with Team Rocket. Of course, they take the bag and Farfetch'd and leave with a boat which was conveniently left behind nearby. However, the bag is full of rocks and the boat has a hole in it. Farfetch'd collects Team Rocket's Pokéballs from the water and takes them back to his trainer.
Misty explains what happened to officer Jenny, and it appears that she isn't the only one who has been robbed: She's the fifth victim of the thief this week. Misty shows Jenny where it happened, and when they arrive there Psyduck shows up, who managed to get away from the thief. Meanwhile the thief is confronted by Team Rocket, but he manages to dupe them once again: He gives them a bag full of Pokéballs. But when they leave, Team Rocket finds that they aren't Pokéballs, but Voltorbs!
The thief is about the make a break for it when he is surrounded by Ash, Misty, Brock and Jenny. They challenge him to a Pokémon fight, but the boy is convinced that Farfetch'd is too weak to fight. But Farfetch'd does want to fight. Ash sends out Bulbasaur, and his Vine Whip causes quite some damage. But Farfetch'd strikes back with Agility and multiple Leak Slaps, defeating Bulbasaur.
The boy says that if he had known Farfetch'd was that strong, he'd never have used him to steal in the first place. Misty wants to fight as well, but the only Pokémon she still has at that moment is Psyduck. He takes a beating from Farfetch'd's Leek Slaps, but when his headache has gotten bad enough, Psyduck uses Disable to defeat Farfetch'd.
After that, (and after Team Rocket bombs him with Voltorbs) the boy agrees to give back everything he stole, and train his Farfetch'd to fight instead of steal.