Ash has reached the Pastoria City Gym, but there's no Gym battling today—the Gym Leader, Crasher Wake, is off to the Pastoria Great Marsh. He invites Ash and the gang to join him, and our heroes are spooked to see lots of Croagunk emerging from the marsh grass. Every household in Pastoria City has a Croagunk and today is the annual Croagunk Festival! Last year's festival honors went to Craig, a Croagunk owned by Hamilton. Hamilton wants Craig to marry Chrissy, Nurse Joy's Croagunk, but Chrissy isn't interested. Hearing this, Brock enters the Croagunk Festival, hoping his Croagunk will win the crown and save Chrissy from an unwanted union.
Out in the marsh, James reminisces about meeting his Carnivine while Jessie and Meowth plan to enter the Croagunk Festival with Meowth in a Croagunk costume! Meowth even excels in the first round of the Festival, a Croagunk beauty contest that leaves Dawn at a loss to tell all those Croagunk apart. The next round is a brick-breaking competition; Brock's Croagunk aces the round, but Meowth ends up with nothing more than an aching paw to show for it. So now it's Brock's Croagunk and Hamilton's Craig in the final championship battle!
The title bout is interrupted by Team Rocket's giant Croagunk Robot, which steals the Crown and both battling Croagunk. Crasher Wake left his Poké Balls at the Gym, so he calls on all the other Croagunk to stop Team Rocket. Nurse Joy and Chrissy lead the charge with Chrissy's Vacuum Wave attack, and with a little help from Ash and Pikachu, Team Rocket is put down for the count. Chrissy's show of bravery earns her the Croagunk Festival crown, and everyone celebrates by singing and dancing into the night. But come the next morning, Ash will have to face Crasher Wake at the Pastoria Gym!
Ash and his friends arrive at Pastoria City and run into Crasher Wake. He introduces himself and Ash immediately challenges him to a Gym battle. Crasher Wake says he has some business to attend to and asks Ash and his friends if they would like to accompany him; they agree.
They hurry to catch the tram that takes them through the Great Marsh. When they arrive, Crasher Wake rushes ahead and they arrive at an open field at sunset. They're startled by glowing eyes in the bushes, which turn out to be a bunch of wild Croagunk. Then, they follow a bunch of Croagunk Trainers to a giant Croagunk Temple. They meet up with Crasher Wake again, who informs them of the Pastoria Croagunk Festival and the ancient tradition of battling Croagunk, with the leader becoming Champion of the Great Marsh.
Brock intrudes in an argument between Nurse Joy and Hamilton, a brown-haired (turquoise-haired in the first airing of the episode in Japan) male whose Croagunk, Craig, won last year's Croagunk Championship. Brock goes crazy for Nurse Joy, proposing a marriage of their Croagunk, only to be Poison Jabbed by his own Croagunk and dragged off-screen.
Meanwhile, Team Rocket arrives at the Great Marsh. James recalls his childhood, during which he first met and rescued his Carnivine, who was trapped in the Great Marsh mud.
Ash and his friends enjoy the evening festival, eating Croagunk cookies and watching Croagunk-shaped fireworks. Crasher Wake referees the event and introduces Hamilton, the defending Champion. Crasher Wake, the operator of the rail system, and Nurse Joy judge the competition. All of the Croagunk take turns coming on stage, crouching then croaking, apparently comparing their looks and physical condition. The judges and local crowd are excited by their different aspects, but Ash and company are confused; as far as they can see, the Croagunk all look exactly the same. Brock's Croagunk takes the stage and the judges are impressed by Croagunk's looks.
Team Rocket dresses Meowth up in a Croagunk costume and sends him out on stage. Meowth is thoroughly embarrassed, but the crowd and judges go wild for it. Hamilton is unimpressed.
Crasher Wake announces the brick-breaking contest. Brock's Croagunk uses Brick Break and breaks all of the bricks. Meowth takes the stage next, to high anticipation. He charges up and attacks the bricks, but since he can't learn Brick Break, the only thing he breaks is his paw. Crasher Wake announces a championship battle between Brock's Croagunk and Craig. They take to the boxing ring and begin the match. Craig uses Cross Chop. The dueling Croagunk trade jabs and Dawn has trouble figuring which one is which.
Out of nowhere, Team Rocket storms the festival in a giant Croagunk robot. Its tongue steals the prized crown. Craig and Brock's Croagunk attack the robot, but are trapped by the tongue. Crasher Wake steps in to help, but suddenly remembers that he left all his Poké Balls at the Gym. Everyone else's Croagunk attack the giant robot, but they're scooped into its belly. Chrissy heavily damages the robot, followed by Pikachu's Thunderbolt.
Crasher Wake retrieves the crown and asks the crowd if they agree that Nurse Joy's Croagunk won this year's championship, which they do. Afterwards, Crasher Wake tells Ash that he was impressed by Pikachu's Thunderbolt and accepts his challenge for a Gym Battle, but it'll have to be held the next day as there's still plenty of partying to do. And so, with Ash's fourth Sinnoh Gym Battle on the horizon, everyone finishes out the traditional Croagunk Festival evening with dancing.