Ash sees a bunch of people lining up. He asks his mom where everyone is going. She tells Ash that they are going for a little trip on Seafoam Island. Even Misty and Brock are going on the trip. Ash wants to go too but his mom tells him that he has to start his training at home.

Ash goes to Professor Oak's house to visit. Professor Oak is on the computer trying to solve something on Pokémon evolution. Professor Oak tells him that if a Slowpoke and Shellder combine, it becomes a Slowbro. Ash suggets for him to look it up in the Pokédex, but the Pokédex doesn't have the answer. Professor oak grabs a hold of his head and tries to think. Professor Oak then gets an idea. He'll ask Professor Westwood for the answer. Professor Oak sees that Westwood lives on Seafoam Island. Ash says goodbye to Mr. Mime and he heads for Seafoam Island.

Ash has arrived at Seafoam Island. He sees Misty and Brock doing some windsurfing. He calls them over and says that he's doing some "special training". Misty and Brock change into their normal clothes. Ash tells them that he's here to meet Professor Westwood. Westwood lets them in his place and he tells Ash and co. to call him Westwood the 5th. Westwood shows them his family pictures. Westwood spots Togepi and takes a few notes on it. Westwood tells them that he's working on a fictional account. Because that means that he's making stuff up, he apologizes to his fathers.

Westwood tells them that he's trying to figure out why Slowpoke evolves into Slowbro when it attaches to a Shellder. Ash comments that Professor Oak is trying to figure that out too. Westwood starts to fish. Ash wonders why and Westwood tells him that he's studying Slowpoke's lifestyle. Ash looks up Slowpoke in his Pokédex. Dexter says that Slowpoke is the dopey Pokémon. They wonder why but Slowpoke doesn't look too smart. Misty knows that she has seen the face expression before. Psyduck comes out and Psyduck and Slowpoke talk to each other. A Krabby comes out and it pinches both Slowpoke and Psyduck. Psyduck yells in pain. Slowpoke lifts up its tail and it tries to shake a Magikarp off it. Westwood invites them all for lunch.

Giovanni and his Persian are relaxing at the beach. Jessie and James are digging for clams because that was the boss's order. Meowth is jealous of the Persian. James notices that everyone else if having fun. This gets James a bit disappointed and hit pulls out a Shellder. Then James says, "Weezing, I choose you!" Jessie notices that James is acting like Ash more and more. Weezing heads straight for Shellder but Shellder fights back. Shellder uses Ice Beam but it misses Weezing. Then Weezing uses Haze and Shellder is stunned. James takes out a Pokéball to capture Shellder. However, Jessie captures Shellder first. One of Giovanni's assistant brings them to Giovanni. Giovanni tells Jessie, James, and Meowth to break into Westwood's laboratory. If they mess up, then they will get into some serious trouble. Jessie, James, and Meowth agree and they take the job.

Ash and co. are having lunch. Westwood takes out a computer and shows them the problem. The problem has to do with the shapes to the tails. Ash sees a parasail heading straight their way. Team Rocket is on the parasail and they crash right into the spot where Ash and co. are. Then Team Rocket says their motto. When they are finished with the motto, the ask Westwood if he's the guy that programmed the Pokédex. Westwood says yes and Jessie hands him a paper. Westwood reads the paper. The paper just states Team Rocket's bios. Team Rocket tells Westwood that they want his Pokémon, but Westwood only has a Slowpoke. Westwood picks up Slowpoke and he runs away with Team Rocket and Ash and co. chasing him.

Jessie sends out Shellder and Shellder chases Slowpoke in circles. Misty wants to battle with Staryu but Psyduck pops out again. Psyduck bites on its tail. Ash thinks that it will become a "Psybro" but there's no such thing. Psyduck shakes Shellder off. Weezing pounds on Psyduck's head. Misty tells Psyduck to run away but Psyduck just trips over. Jessie throws Shellder towards Slowpoke but Shellder ends up biting Psyduck's head. Psyduck gets a headache and it disables Team Rocket. Then Psyduck lifts Team Rocket off the ground. Psyduck's psychic energy runs out and Team Rocket falls down. Slowpoke walks towards Psyduck. It trips on Shellder and Shellder bites Slowpoke's tail. Slowpoke evolves into Slowbro. Westwood takes lots of notes on Slowpoke's evolution.

Team Rocket runs towards Slowbro so that they can capture it. Westwood tells Slowbro to use Mega Punch on them, but Slowpoke is suffering from Amnesia. Then Slowbro remembers what it's doing and it uses Mega Punch on Team Rocket. Team Rocket goes blasting off again.

Westwood now has figured out why a Shellder clamps on a Slowpoke's tail. The reason is because it helps both Pokémon out. Westwood explains the process. Shellder helps Slowbro balance itself when it stands on two feet. Because of this, Slowbro's front legs are free and the it can use attacks such as Mega Punch. Now Shellder can travel on land. Psyduck wants to say goodbye to Slowbro before they leave. Slowbro and Psyduck just stare at each other and talk.

English Official Summary

Ash decides to join his mom and friends who are vacationing on Seafoam Island. While there, he searches out the programmer of the Pokedex to get information about the evolution of a Slowpoke. The mystery of how Slowpoke evolves into a Slowbro is surprisingly revealed when Team Rocket makes their usual bumbling interference.

Italian Official Summary

Ash raggiunge la mamma e amici in gita alle Isole Spumarine (Seafoam Islands) per visitare uno degli esperti programmatori del Pokédex e cercare di risolvere il mistero dell'evoluzione degli Slowpoke. L'intervento rocambolesco del Team Rocket chiarisce a sorpresa l'enigma!

Portuguese Official Summary

Ash junta-se à mãe para umas férias na Ilha Espuma. Lá, ele procura o programador do Pokédex para saber mais sobre a invulgar Evolução de Slowpoke. Este mistério é surpreendentemente revelado pela balbuciação da Team Rocket!

Spanish Latin America Official Summary

Ash se une a su mamá en sus vacaciones en la Isla Espuma. Ahí, busca al programador del Pokédex para aprender de la inusual Evolución de SlowPoké. ¡El misterio es revelado por la incompetencia del Equipo Rocket!

Spanish Official Summary

Ash decide reunirse con su madre y amigos que están de vacaciones en las Islas Espuma.

English Great Britian Official Summary

Ash decides to join his mom and friends who are vacationing on Seafoam Island. While there, he searches out the programmer of the Pokedex to get information about the evolution of a Slowpoke. The mystery of how Slowpoke evolves into a Slowbro is surprisingly revealed when Team Rocket makes their usual bumbling interference.

Dutch Official Summary

Ash is met zijn moeder op Seafoam Island. Daar zoekt hij de programmeur op van de Pokédex, om iets te leren over de ongewone Evolutie van Slowpoke. Dit mysterie wordt echter opgelost door het gestuntel van Team Rocket!

French Official Summary

Sacha décide de rejoindre sa mère et ses amis qui passent les vacances sur l'Île Écume.

German Official Summary

Ash schließt sich seiner Mutter und ihren Freundinnen an, die auf den Seeschauminseln Urlaub machen.

Danish Official Summary

Ash tager på ferie med sin mor til Seafoam-øerne. Der opsøger han Pokédex-programmøren for at lære noget om den sjældne Slowpokes Udvikling. Dette mysterium bliver på mystisk vis afdækket af Team Rockets kludren!