Ash and his friends have stopped to rest at a mountain stream. All of the Pokémon are in the water, having a good time. Brock is brushing Vulpix's tail, while Misty watches Togepi splashing in the lake. Poliwhirl drifts by on its back, and Misty tells it not to drift too far. Poliwhirl waves okay, but all of a sudden, a huge Poliwrath comes up behind it. As Misty and Brock anxiously watch, Poliwrath smacks Poliwhirl out of the water. As Misty attends to her injured Poliwhirl, a young Trainer called Andreas appears and insults Misty and her Poliwhirl. Poliwhirl then hides behind Misty’s legs. Misty defends herself and Poliwhirl, as do Ash and Brock, but Andreas continues to insult her, taking it a step further by also insulting Staryu, Goldeen, and Psyduck. Misty angrily challenges him to a battle, but Andreas quickly recalls his Poliwrath and admits that he already has other plans.
Misty is still annoyed by Andreas's earlier comments, but Brock reassures her that Poliwhirl is a great Pokemon. The group starts walking down a trail when they notice a large mass of people with fishing rods passing by. Ash asks a boy if there is a good fishing spot nearby and the boy responds that there is an annual Seaking Catching Competition. The group soon joins the large crowd of competitors. At the site of the actual event, a man onstage starts to explain the rules of the competition. Competitors are allowed to use any rod, reel, or lure they wish and can catch as many Seaking as they want in the allotted time. However, they are only allowed to enter one Seaking that was caught in the provided Lake Ball. The Seaking will then be weighed, and the heaviest one will win a huge trophy and a year's supply of chocolate bars. Competitors are only allowed to use one Pokémon to aid them in catching the Seaking, and it must be registered. Ash remarks that it is similar to the Bug-Catching Contest, and decides to use Pikachu. At the registration line, Misty notices that Andreas and his Poliwrath have entered the competition, and despite the rumors that he is unbeatable, Misty also decides to compete in the event. However, Brock is more interested in the female registration attendant and Misty drags him away by the ear.
At the docks, all the competitors are in their boats and waiting to begin. Brock takes the wheel and asks Ash and Misty if they're all ready. They respond yes as Andreas pulls up. He insults Misty and her Poliwhirl again. Misty says that Poliwhirl isn't weak just because it isn't a bully like him. Andreas laughs at her and says that he'd love to talk, but he has a competition to win before rushing off. The horn blows and the competition has begun. Ash and Misty both cast out. Misty tells Poliwhirl that it'll probably take a while. She then hears Andreas yell that he already has a bite, to Misty's dismay. He pulls the Seaking out of the water, and has Poliwrath use Double Slap before making a swift capture. Andreas then releases the Seaking that he just caught, much to Ash and Misty's surprise. He tells them that it was just a warm-up, and the next catch will be really big, which angers Misty even more.
Meanwhile, Team Rocket is in a boat. Meowth says that if they win the contest then they will get a year's supply of chocolate bars. James says that they haven't eaten in so long that he'd settle for licking the wrappers. Jessie shows them her new lure, a chibi-style Jessie, saying that no Seaking can resist it. After some time, Meowth yawns and complains that they've been sitting there for an hour and maybe Jessie's lure is scaring off all the Seaking. Jessie gets angry and says that the Seaking obviously don't appreciate her beauty. Jessie then says that if Seaking won't go for a beautiful lure, then they'll go for an ugly lure; a Meowth lure. Meowth says that he didn't know she had a Meowth lure but soon learns that he is the fishing lure. Jessie puts a breather in his mouth, ties him to a rock, and throws him into the lake, telling him to use Scratch when he sees a Seaking. Underwater, Meowth complains that he doesn’t like it and that he can't see any Seaking. One swims past him and he yells at it to come back, but it ignores him. Suddenly, Misty's line wraps around Meowth's arm and pulls him up. Jessie pulls back, turning it into a tug-of-war match with Meowth in the middle. Underwater, the rock tied to Meowth gets stuck to the bottom, immobilizing him. As if that wasn't bad enough, he then loses his breather. The two boats are being dragged towards each other until they collide. Back underwater, Meowth uses Fury Swipes to free himself and swims to the surface. Team Rocket drives away and Misty sees that her line is broken. Andreas drives by and mocks her on her performance. He tells her that she could take home first prize in a fishing rod catching competition, continuing. Meanwhile, Meowth gets back on Team Rocket's boat, saying that that was a bad idea.
Misty observes that the wind has changed direction and she urges Brock to drive the boat northwards. Misty then notices Andreas speeding towards the same place, so she takes the controls of the boat and rushes on. She and Andreas start racing each other. They get into a bumping fight and are speeding towards an unsuspecting Team Rocket. Brock tells Misty to turn and try avoiding their boat, but Andreas uses it as a ramp and flies right over them. The resulting wake causes Team Rocket's vessel to capsize and they enter the water. Misty drives over to see Team Rocket are okay. To remain undetected, Jessie and James duck behind their boat and wave the group off.
The gang arrives at a giant rock. Misty says that it looks like a perfect spot for fishing, but Andreas is already there. Ash gets a bite and pulls up a big Seaking. He commands Pikachu to use Thunderbolt on it, and catches it in a Lake Ball. Misty and Brock compliment him on his catch. Andreas then gets a bite as well. He has Poliwrath get ready and then pulls it out. Poliwrath uses Mega Punch on Seaking, followed by a Double Slap. Seaking flies out of the water and Andreas catches it, bragging that it is a guaranteed winner. The loudspeakers come on and the voice says that only ten minutes are left in the competition. As Andreas is driving away, he says that if Misty wants a chance at second place then she better hurry.
Misty announces that it's time to use her secret weapon, her special lure version 4. She throws out her lure and waits intently, and a giant Seaking soon bites it. It almost pulls her in, but she manages to pull it out of the water. She sends out Poliwhirl, but it gets slapped by Seaking's tail. Poliwhirl flies back and rebounds off the boat to Double Slap the Seaking. Misty catches it in her Lake Ball, as the buzzer rings, announcing the end of the contest. Team Rocket, having not caught a Seaking, walk away with only a commemorative badge. While James is pleased with a memento, Jessie is annoyed and Meowth is disappointed that they won't be winning the chocolate. The announcer weighs Andreas's Seaking, which weighs 55 kilograms, and Andreas brags that he's number one once more. Suddenly, Misty runs on to the stage, yelling for the judge to wait. They weigh her Seaking, which turns out to weigh the same as Andreas's, meaning that the two of them must face off in a Pokémon battle to determine the winner.
The stage is set for a Water-type battle, and Misty sends out Poliwhirl to go up against Andreas and his Poliwrath. Brock comments that the odds are against Misty. Just as the match begins, Team Rocket descends from the sky in their Meowth balloon. Jessie sends Meowth out on a fishing pole, and he starts bagging all of the chocolate. Misty commands Poliwhirl to attack him with Water Gun and Andreas commands Poliwrath to do the same, causing Meowth to drop the loot. Ash sends out Bulbasaur and has it use Vine Whip to catch the falling chocolate. With the chocolate safe, Ash has Pikachu give them a Thunderbolt, sending them blasting off again.
The battle resumes, and Andreas has his Poliwrath use Submission, which sends Poliwhirl flying. Ash is surprised, but Brock explains to him that after a Poliwhirl evolves, it can use Fighting-type moves. Andreas is quick to claim the victory, but Poliwhirl gets back up and hides behind Misty's leg. Misty says that she knows it's doing its best, but wishes that it had more confidence in itself. Psyduck, Staryu, and Goldeen pop out of their Poké Balls and Misty tells Poliwhirl that there are lots of great unevolved Pokémon and that it's one too. With the pep talk, Poliwhirl is pumped up and ready to get back in the battle. Andreas says that it's no use, but Misty has Poliwhirl use Bubble and then follow up with Mega Punch. Poliwrath falls, but Andreas retaliates with several Mega Punch attacks. Poliwhirl dodges them all and uses Bubble again, followed by a Water Gun that knocks Poliwrath out. The announcer officially declares Misty the winner of the match and the Seaking Catching Competition. Misty and Poliwhirl are dancing, and she tells it she knew it wasn't a wimp. Andreas pulls Poliwrath out of the water and congratulates Misty on a job well done. He then apologizes for the things he said, and Misty accepts his apology. Misty is pleased with herself and later sets herself the task of dividing her large chocolate prize between her friends and family, though Ash soon complains that he didn't get any.