Ash is surprised to learn that his mother Delia, some of Pallet Town's locals, and his friends are going to go to Seafoam Island for a beachside holiday. Delia tells Ash that he has to stay behind to train, just as the tour group leaves shortly after. Ash goes to Professor Oak's lab, only to discover that Oak is trying to find out why Slowpoke evolves into Slowbro. When Ash suggests that he use the Pokédex to search for answers, Oak is instantly reminded of Professor Westwood V, a Pokéologist and the original creator of the Pokédex who would know the answer. He conveniently lives of Seafoam Island, so Ash and Pikachu quickly decide to assist, capitalizing on an excuse to go to the beach.
Ash arrives and surprises Misty and Brock. They walk to Westwood's lab, where they discover that he is actually the fifth of many Westwoods, and insists on being called that. He is also trying to figure out the mystery of Slowpoke and Shellder, and hopes to present a theory to the world's leading body of Pokéologists, the Pokémon Suppositum. Westwood V goes outside to fish, trying to understand Slowpoke. As Ash and his friends find a Slowpoke sitting next to them, Psyduck comes out of its Poké Ball and talks to the Slowpoke. After what seems to be some random, unintelligent conversation, a Krabby comes out the water. It first grabs onto Slowpoke's tail, and then Psyduck's, which the two register several seconds later. The team decides to go have some lunch for a break.
Meanwhile, Team Rocket's Boss is taking a vacation on the beach to rest and recover from the stress of having Team Rocket's headquarters destroyed and making Jessie and James dig for clams, having already cleaned his private yacht. Upset that they cannot take a break, James digs up a Shellder and decides to try to capture it. James calls out Weezing, who, after dodging Shellder's Ice Beam, defeats it with Haze. However, just as James attempts to capture the Shellder, Jessie butts in and captures it herself, much to James's disappointment. They go to the Boss, but he disapproves of their capture and orders them to steal potentially invaluable information from Westwood V's lab.
Westwood V shows Ash and his friends a computer simulation of Slowpoke's evolution, and notes the form change Shellder undergoes when it clamps on to Slowpoke's tail. Team Rocket arrives in a parasail and startles everyone when it crash-lands on the lab's balcony. After performing their motto, they hand Westwood V a paper with their own "Team Rockédex" entries about themselves. Westwood V is instantly impressed by Meowth's speech and starts writing notes. He then realizes that Team Rocket may be trying to enslave him and tries to escape. The trio finds that the only Pokémon Westwood V has is a Slowpoke, and decide to use their new Shellder to make Slowpoke evolve into Slowbro and thus make it more valuable.
Westwood V tries to flee with his Slowpoke, but after tripping, Team Rocket catches up and sends out Shellder. To everyone's surprise, Slowpoke, regarded as the Dopey Pokémon, runs around in an attempt to flee from Shellder. Misty decides to intervene with the seaside location perfect for her Water-type Pokémon, but Psyduck emerges. Psyduck turns around and Shellder clamps onto its tail instead. After a delayed pain reaction, Psyduck throws Shellder off, sending the Pokémon flying. Weezing attempts to knock Psyduck out, but only bounces on its head. Infuriated, Jessie throws Shellder again, but it clamps onto Psyduck's head this time, giving it a major headache. Psyduck, now much more powerful because of its headache, uses Disable to stop everyone and its Confusion sends Team Rocket flying. It suddenly loses concentration, however, both Team Rocket and Shellder land close by. As Slowpoke walks towards Psyduck to thank it, it trips and Shellder clamps onto its tail, evolving it. Team Rocket runs back to grab Slowbro, but Westwood V orders Slowbro to attack them with Mega Punch. Although Slowbro temporarily forgets the attack due to Amnesia, it quickly regains it and sends Team Rocket blasting off once again.
Misty asks if Westwood V figured out why Slowpoke evolves. He answers confidently, with a brand-new theory, that both Pokémon benefit. He explains that Shellder helps Slowpoke to balance on two legs instead of four, freeing its hands so it can use Mega Punch, and Shellder gains land transportation on Slowpoke's tail. As the group leaves, Ash thinks Psyduck may want to say goodbye to Slowbro. Misty, Ash, and Brock watch on as the two Pokémon have a rather slow conversation.