Ash goes outside his house to see a group of people from his neighborhood about to go somewhere. His mother tells him that they are all going to Seafoam Island. Misty and Brock wave to him from the line, and tell him that they were invited as well. Ash starts to go with them, but his mom grabs him by the collar, telling him that he has training to do. They all leave, leaving Ash alone with Mr. Mime. Ash gets bored, and heads over to Professor Oak's lab. He sees the professor clutching his head, and asks him what's up. He tells Ash that he's being trying to figure out the mystery behind Slowpoke's evolution into Slowbro. He tells Ash that the Pokédex doesn't have the answer to the mystery, but then gets an idea. He tells Ash that a Professor Westwood, the programmer of the Pokédex, might have the answer. As he lives on Seafoam island, Ash sets off.
Upon reaching the island, he sees Brock and Misty windsurfing and waves to them. They're both surprised to see him there, and Misty falls off her board. A few minutes later, Brock and Misty walk with Ash to Professor Westwood's lab. He lets them inside and shows them around. He tells them that he is the fifth of a long line of Westwoods, and asks them to call him just that: Westwood the 5th. He looks at Togepi, and raves over how he's never seen one like him. He tells them that he only has information on rare Pokémon that he's made himself. He is, in fact, working on a fictional account at the moment, and apologizes to his forefathers. He then explains to Ash and his friends that he's trying to figure out the mystery behind Slowpoke. Ash says that Professor Oak couldn't figure that out either, and Westwood tells them that he needs to come up with a theory for the Symposium: a group of Pokologists.
They go outside, and Westwood starts to fish. Ash asks him why he's doing this, and he explains that Slowpoke is one of the few Pokémon that fish. Ash uses his Pokédex, which refers to Slowpoke as a Dopey Pokémon. Westwood tells them that, while dopey may not be very nice, it pretty much fits Slowpoke's mental capacity. Misty asks Slowpoke if he's going to defend himself, but he just stares out to sea. Misty notes that the expression on his face looks familiar and, as if on cue, Psyduck pops out of his Pokéball. Psyduck approaches Slowpoke, and they start to talk, sort of. A Krabby comes out of the water, and pinches Slowpoke's tail. He doesn't react, and Krabby does the same to Psyduck. It takes a few seconds, but Psyduck eventually yells out in pain. Slowpoke lifts his tail, as he's caught a Magikarp. Westwood invites them back inside for some lunch.
Meanwhile, down the beach a ways, the Team Rocket Boss relaxes outside his mansion, while Persian licks from a saucer of milk. A little ways away, Meowth starts to tear up as he looks at the wonderful treatment Persian is getting. Jessie and James are nearby, digging in the sand. James complains about how the Boss has been making them do things like swab his yacht and dig for clams. He looks out at the people having fun on the ocean, and starts clawing at the sand in anger. Suddenly, he hits something and pulls it out. It's a Shellder, and James attacks it with Weezing, but Weezing is knocked away. Shellder then uses Ice Beam, just missing Weezing. Weezing nails Shellder with a Haze attack, and James readies a Pokéball. Jessie steps in, however, catching Shellder herself. Their boss calls them over, and tells them how upset he is with their constant failures. He tells them to break into Professor Westwood's laboratory, and steal the Pokémon there. They hurriedly leave.
Back at the laboratory, Misty asks Westwood what exactly the mysteries behind Slowpoke's evolution are. He tells them that before a Shellder clamps onto a Slowpoke's tail, it has a hinged shell. Afterwards, Shellder gets a spiral shell. He imagines being a world-famous scientist, and Ash and his friends stare at him. Suddenly, they hear the sound of a motorboat, and look to the ocean to see a parasailer flying around. The rope detaches, and the parasailers float over to the lab, crashing onto the deck. They thrash about underneath the tarp for a few seconds, and Westwood asks what's going on. At this, Team Rocket does their motto, tearing off the parachute. They hand Westwood a piece of paper, and he reads off three mock Pokédex entries for Jessie, James, and Meowth. Misty crumples up the paper, and Westwood starts to write about the talking Meowth. It dawns on him that Team Rocket is here for no good, and dashes into the lab. Team Rocket informs him that all they want is his Pokémon, but he says that Slowpoke is the only one he has. Slowpoke walks in, Team Rocket looks at him. Meowth tells Westwood that they'll just evolve Slowbro with Jessie's Shellder, and take him to their boss. Westwood picks up Slowpoke, and dashes outside. He trips, however, and Slowpoke falls to the ground. Jessie sends out Shellder, who chases the now quite fast Slowpoke around in circles.
Misty prepares to send out Starmie, but Psyduck jumps out instead. Shellder clamps down onto Psyduck's tail, who, after a few seconds, runs around in pain. James sends out Weezing, who starts bouncing on Psyduck's head. Jessie throws Shellder at Slowpoke, but he ducks and Shellder clamps onto Psyduck's head. Psyduck yells in pain for a moment, then stops. His eyes start to glow, and Brock points out that Psyduck only uses his special attacks once his headache gets really bad. Psyduck paralyzes Team Rocket with a Disable attack, then floats all of them into the air, including Shellder, with Confusion. Westwood starts to write in his little notebook, when Psyduck suddenly stops. Team Rocket fall to the sand, and Shellder bounces off Psyduck's head. Westwood tells Slowpoke to thank Psyduck, but he slips on Shellder's shell, and Shellder clamps onto his tail. Slowpoke evolves into Slowbro, and Westwood tells Ash and his friends that he is the first researcher to see this happen. Team Rocket runs up to take Slowbro away, and Westwood tells him to attack them. He tells Ash and his friends that Slowbro knows a powerful move called Mega Punch, and commands Slowbro to use it. Unfortunately, Slowbro appears to be suffering from his own attack of Amnesia, and doesn't remember it. Slowbro remembers, just in the nick of time, and punches Team Rocket over the horizon.
Westwood starts to furiously writing in his notebook again, and tells Ash and his friends that he's figured out the mystery. He tells them that it's "mutually beneficial," meaning it's helpful for both Pokémon. He tells them that a Shellder's weight helps balance Slowbro's body out, giving it the ability to use it's upper appendages. Shellder can now travel on land since he's clamped onto Slowbro's tail.
Later, Ash and his friends leave the lab, and Misty tells Ash that he wasted a whole day of training. Ash tells her, however, that this was training. Misty notices that Psyduck is looking dejected, and asks him what's wrong. Ash figures that it's because he wanted to say goodbye to Slowbro, and they go back. They sit and wait as the two Pokémon talk to each other. Judging by the amount of things they're saying, they're going to be waiting a while.