Ash’s mom and friends are going to go to the 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, the original creator of the Pokédex who would know the answer.
Happy to have an excuse to go to the beach, Ash and Pikachu head to the Seafoam Islands. Ash arrives and surprises Brock and Misty, who are riding on sailboards. They walk to Westwood’s lab and discover that he is actually the fifth of many Westwoods, and insists on being called that. He happens to also be trying to figure out the mystery of Slowpoke and Shellder. Westwood goes outside to fish, trying to understand Slowpoke. Ash and his friends examine Slowpoke sitting next to them. Misty's 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 the water. It first grabs onto Slowpoke's tail, and then Psyduck's, leaving it in great pain jumping up and down. The team decides to go have some lunch for a break.
Meanwhile, Team Rocket's Boss is taking a vacation on the beach and making Jessie and James dig for clams. Upset that they cannot take a break, James digs up a Shellder and decides to try to capture it. James calls out Weezing and defeats the Shellder with Haze, but Jessie steps up to capture it. They go to the boss, but he is annoyed with them as usual. He orders them to steal all the Pokémon from Westwood's lab. The Team Rocket members arrive in a parasail and startle everyone, especially Professor Westwood, who tries to escape. They find that the only Pokémon Professor Westwood has is the Slowpoke. Westwood tries to run away with his Slowpoke, but Team Rocket catches up and sends out the Shellder to make it evolve into Slowbro. They have a hard time with it, and it accidentally clamps onto Psyduck’s tail instead. Jessie gets mad and throws the 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 causes Team Rocket to fly off into the distance with Confusion. As Slowpoke walks toward Psyduck to say thanks, it trips and Shellder clamps onto its tail, evolving it. Team Rocket runs back to grab Slowbro, but it uses Mega Punch, causing Team Rocket to blast 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 land transportation on Slowpoke’s tail. The episode ends as Psyduck goes to say goodbye to Slowbro, though it only looks like meaningless conversation.
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