Ash, Misty, Brock, and Pikachu are passing through Rikishii Town when they see a set of Feraligatr matches taking down a few Pokémon. After its sixth win, Misty takes a closer look at the Feraligatr and quickly falls in love with the Water type. The gang soon finds out that the Feraligatr is owned by a Trainer named Raiden, who tells them that the competitions here use sumo-wrestling and the Pokémon Sumo Competition is being held by Shonosuke, the director of the Pokémon sumo society. Shonosuke explains the rules of Pokémon sumo-wrestling, which the Pokémon only has to rely on their physical skills and not using any of their usual attacks. He even mentions that Raiden and his Feraligatr are expecting to win this year’s Pokémon sumo competition, which is being held today. Ash decides to participate in the competition but Shonosuke reminds him that he needs to have a Pokémon that is heavy. Pikachu enthusiastically volunteers, but Ash gently turns the offer down, reminding him that he won't pass the weight requirement.
At the Pokémon Center, Ash asks Professor Oak to transfer his Snorlax due to the regulations of the competition. Since Ash has six Pokémon, Ash transfers his Noctowl because Professor Oak wants to do research due to its unusual rare coloring. At the registration area, a disguised Team Rocket enters Wobbuffet, disguised as a Miltank nicknamed "Milky", into the competition. James and Meowth are concerned about Jessie's plan of entering her Wobbuffet since it's a light-weight Pokémon, but Jessie tells them not to worry as she has already planned everything out. Meanwhile, Ash manages to get his Snorlax into the competition, being the last person to register. It's just as well, as half of Snorlax's weight breaks the weigh-in scale. Snorlax quickly falls back to sleep, even missing the opening ceremony. But as Shonosuke announces the prize, which is the King's Rock and a one year supply of Pokémon food, Snorlax finally wakes up when it hears about the food prize.
After Shonosuke finishes his speech and the rules, the gang meets up with Raiden and his Feraligatr and they were amazed by the opponent's big Pokémon especially the Blastoise whom everyone says that it might win this competition. The first match is between Jessie's "Miltank" versus an Ursaring. Jessie makes an easy win and was announced the winner of the match, but Shonosuke lodges a protest to the referee. Shonosuke noticed that Jessie's "Miltank" used a Counter attack and the official disqualifies them. Not too happy about his decision, they reveal their true identity, even their Wobbuffet. Shonosuke is furious and tells them that they made a mockery of the rules of this competition. Not bothered by what he says, Team Rocket is about to send out their Pokémon to attack but they were sent blasting off again by a Hydro Pump from Raiden's Feraligatr.
The next match is between Raiden's Feraligatr versus a Golem. The gang assumes that Golem is going to win due to its weight advantage, but Raiden turns the tables by using Golem's disadvantages, knocking Golem down to the ground and winning the match. Ash was amazed at his match and learns from it. After a few matches, it was Ash and Snorlax's turn. Snorlax goes up against a Poliwrath and as the match begins, Snorlax makes a surprising win as the Poliwrath attacks Snorlax's stomach and was bounced back, quickly knocking it out. The shocked referee announced the win and Snorlax happily goes for the prize, but Ash quickly manages to call Snorlax back to its Poké Ball. Raiden and Shonosuke were amazed by Ash's Snorlax's ability since Ash is just a rookie. Outside the arena, Team Rocket cooked up a plan by selling phony Pokémon paw prints, supposedly from the Pokémon who are participating in the competition to the audience for money.
Soon, both Ash and Raiden make it to the semi-finals. The two matches are Raiden's Feraligatr against Blastoise while Ash's Snorlax fights Machamp. While Feraligatr wins in a very close match, once again Ash's Snorlax makes an easy win via its stomach, much to Ash's surprise. Raiden congratulates Ash for making it to the finals, but he won't go easy on him just because he's a nice kid. Shonosuke tells Ash that he should be proud of himself; Ash is the first rookie to ever reach the finals. He tells them that finals are next and for them to give it their best.
The final match begins between Raiden's Feraligatr and Ash's Snorlax. Snorlax makes the first move but Feraligatr dodges it. Snorlax barely manages to retain its balance and teetering on the edge of the ring, still manages to dodge Feraligatr's attack. Raiden is immediately shocked and impressed with how deceptively agile Snorlax is. Both of them make an impressive move but as Feraligatr was pushing Snorlax out of the ring, Ash tells Snorlax to take a deep breath, using its stomach to push Feraligatr away, saving Snorlax. Feraligatr manages to toss Snorlax, but Snorlax manages to land in the ring with a huge impact, causing the entire stadium to shake. Because of that, Feraligatr loses its balance and Snorlax manages to push it out of the ring, winning the match and the competition. Misty and Brock are happy that Ash wins, even though Misty was sad that Feraligatr lost.
The prize-giving ceremony begins as Ash was presented the King's Rock and a one year supply of Pokémon food. Happily, Snorlax rushes towards it, eats the entire supply of Pokémon food and goes to sleep. The gang was glad that they don't have to carry the supply with them and Ash returns Snorlax into its Poké Ball. Ash decides to send back his Snorlax to Professor Oak and get back his Noctowl since he can't spare enough food for Snorlax. Both Raiden and Ash shake hands and Raiden wishes him good luck on all his Johto League matches.
In the end, Team Rocket, with a bag of phony Pokémon paw prints, notice the competition is already over and have to wait for next year to sell it.