Ash's mom and friends are going to go to Seafoam Islands. Misty and Brock are going to go surfing. Ash goes to Oak's lab to discover that Oak is trying to find out why Slowpoke evolves into Slowbro. Professor Oak suggests that Ash go to the Seafoam Islands to meet Professor Westwood who made the Pokédex who would know the answer. Ash and Pikachu head to the Seafoam Islands and startle Brock and Misty into falling off their surfboards. They go to Westwood's lab to discover he is the fifth of many Westwoods, and he insists to be called that. He happens to be researching the Slowpoke and Shellder, too. Somehow, a Shellder bites Slowpoke's tail and it becomes a Slowbro. Meanwhile, the Team Rocket boss is taking a vacation and making Jessie and James dig for clams. They find a Shellder and James decides to try to capture it. James copies Ash to call out Weezing. James defeats the Shellder, but Jessie steps up to capture it. They go to the boss, but he is annoyed with them as usual. He tells them to steal all the Pokémon from the lab. They find that all he Professor Westwood has is a Slowpoke. They send out the Shellder to make it evolve into Slowbro. They have a hard time with it, and it clamps on Psyduck's head, giving it a major headache. Psyduck uses disable to stop everyone, but then the Shellder clamps onto Slowpoke's tail evolving it. Slowbro uses mega-punch and so Team Rocket blasts off again. Westwood figures out why Slowpoke evolves, which is because both Pokémon benefit. Shellder balances Slowpoke so it can use mega-punch, and Shellder gains transportation on Slowpoke's tail.
A young boy named Ash Ketchum embarks on a journey to become a "Pokemon Master" with his first Pokemon, Pikachu. Joining him on his travels are Brock, a girl-obsessed Rock Pokemon Trainer, and Misty, a tomboyish Water Pokemon Trainer who may have a crush on him. Ash and Co. end up traveling through various regions, including Kanto, the Orange Islands, and Johto, and then enter the Pokemon League competitions there.
Along the way, they run into many confrontations with Jessie, James, and Meowth, a trio of Pokemon thieves who are apart of an evil organization called "Team Rocket". But every time Team Rocket try to do their evil deeds, they fail thanks to Ash and his Pokemon.