Ash and his friends are passing through Rikishii Town when they see a Feraligatr defeating several Pokémon in a row. After its sixth win, Misty takes a closer look at the Feraligatr and quickly falls in love with the Water type. The group greets Feraligatr's Trainer, Raiden, who tells them that the people of the town specialize in Pokémon sumo wrestling and introduces them to Shonosuke, the director of the Pokémon Sumo Society. Shonosuke explains the rules of Pokémon sumo wrestling, in 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 Conference, which is being held today, with a coveted King's Rock as the main prize. As Feraligatr resumes its training, Ash calls out his Totodile to see his evolved form and declares his interest in participating in the Pokémon Sumo Conference. He plans to have Pikachu and Totodile start training for it, but Shonosuke informs him that in order to participate, he needs a Pokémon that is heavy.
At the Pokémon Center, Ash asks Professor Oak to transfer him his Snorlax due to only Pokémon weighing at least 80 kilograms being allowed to enter the competition. Since Ash has six Pokémon in his team, he transfers his Noctowl to Professor Oak so that he can research 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 he'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 registered into the competition, being the last person to enter, with Snorlax's weight breaking the weigh-in scale. Snorlax is fast asleep, even missing the opening ceremony. However, as Shonosuke announces the prize, which includes 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 explaining the rules, the group meets up with Raiden and his Feraligatr. They are amazed by the other big Pokémon that will be competing, especially Blastoise, which is expected to win the competition. The first match is between Jessie's "Miltank" versus an Ursaring. Jessie makes an easy win and is announced the winner of the match, but Shonosuke lodges a protest to the referee. Shonosuke noticed that Jessie's "Miltank" used a Counter attack, which breaks the rules and means that "Miltank" is hereby disqualified. Outraged by this decision, Team Rocket reveals their true identities. 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 are quickly sent blasting off again by a Hydro Pump from Raiden's Feraligatr.
The next match is between Raiden's Feraligatr versus a Golem. The group 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 is amazed and resolves to win as well. After a few matches, it is Ash and Snorlax's turn, with a Poliwrath as their opponent. As the match begins, Snorlax makes a surprising win as Poliwrath attacks Snorlax's belly and is bounced back, quickly knocking it out. The shocked referee announces the win and Snorlax happily goes for the prize food, but Ash quickly manages to recall Snorlax back to its Poké Ball. Raiden and Shonosuke are amazed by Snorlax's ability since Ash is just a rookie. Outside the arena, Team Rocket develops up a plan to sell phony Pokémon paw prints to the audience for money while looking for an opportunity to steal Pokémon.
Soon, both Ash and Raiden make it to the semi-finals. The two matches are Raiden's Feraligatr against Blastoise and Ash's Snorlax fights Machamp. While Feraligatr wins in a very close match, Ash's Snorlax once again makes an easy win via its bouncy belly, much to Ash's surprise. Raiden congratulates Ash for making it to the finals, but also says he won't go easy on him just because he's a nice kid. Shonosuke tells Ash that he should be proud of himself, as he is the first rookie to ever reach the finals. He tells them that finals are next and for them both 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, even while 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 is pushing Snorlax out of the ring, Ash tells Snorlax to take a deep breath, using its belly to push Feraligatr away, saving Snorlax. Feraligatr manages to toss Snorlax, but Snorlax manages to land within the ring with a huge impact, causing the entire stadium to shake. Feraligatr loses its balance as a result and Snorlax manages to push it out of the ring, winning the match and the competition. Misty and Brock are happy that Ash won, even though Misty is also sad that Feraligatr lost.
The prize-giving ceremony begins as Ash is presented with the King's Rock and the one-year supply of Pokémon food. Happily, Snorlax rushes towards the food, eats it down to the last grub within moments, and promptly falls asleep afterwards. The group is glad that they don't have to carry the supply with them and Ash returns Snorlax into its Poké Ball, deciding to send it back to Professor Oak and call back his Noctowl, not having the food necessary to keep Snorlax fed. Both Raiden and Ash shake hands and Raiden wishes him good luck on all his Johto League matches.
Later, Team Rocket, with bags full of phony Pokémon paw prints, notices the competition is already over, much to their dismay, as they now have to wait until next year to sell them.