Ash and his friends have finally reached Scarlet City, the site of the Whirl Cup tournament. Brock tells his friends to stop arguing over who is going to win as they enter the Pokémon Center to register for the preliminary rounds. The lengthy queue comes as a surprise to Ash and Misty. Brock begins flirting with Nurse Joy, but Misty bumps him out of the way. Ash and Misty then register for the Whirl Cup. Nurse Joy hands the group a map of the tournament grounds, and explains that Ash will be competing in Stadium A while Misty’s first round will be in Stadium B. Joy runs through her database and reveals that two wins will be required to enter the actual Whirl Cup event, as only 64 competitors can qualify.
Afterwards, the group eat some icy poles outside. They are suddenly approached by a young Trainer, Marcellus, who pleads with Ash to battle his Remoraid. Ash quickly agrees to the match, though Brock’s warns him that the preliminaries are only tomorrow. Misty asks Marcellus for further details, and he admits that he wants to evolve his Remoraid into an Octillery. Marcellus shows them a stadium for the Whirl Cup and admits that the Water event will largely be occurring on land, so an Octillery would be perfectly suited. Misty decides to accept the challenge instead, and she calls on her Corsola against Marcellus’s Remoraid. Marcellus commands Remoraid to use Water Gun, which Corsola counters with Mirror Coat, knocking back the Remoraid. Corsola goes for a Tackle but Remoraid dives underneath and Corsola hits one of the pillars. It seems like Remoraid is nowhere to be seen, but soon it is spotted riding atop a wave. Marcellus tells Remoraid to use another Water Gun and this time it knocks Corsola out. Remoraid then evolves into Octillery. Marcellus thanks them and declares that he’ll meet them in the Whirl Cup finals before heading off.
The next day, the preliminary matches begin. Ash has Totodile go up against a Trainer and his Seel. Totodlie's fancy footwork helps it avoid the several Ice Beams, and it knocks Seel out with a single Water Gun attack. Misty's first round goes well, and her Poliwhirl Double Slaps a Magikarp out of the competition. Ash breezes through his second round defeating a Shellder with Totodile. In Misty’s second round, her Corsola manages to Recover from several direct hits from a Seaking. A Tackle attack gives Misty her second win, allowing her to advance into the Whirl Cup tournament. The group reunite back at the Pokémon Center where Misty and Ash reveal they both won their two preliminaries. Marcellus walks passed them and appears quite glum. Misty offers him some advice, and this appears to cheer Marcellus up. He then invites them round to his place for dinner, and the group accept the offer.
Inside Marcellus's mansion, the group ventures into a room with Remoraid in several aquarium tanks. Marcellus explains that he has caught Remoraid from all over including the southern sea. He admits that it is taking his Pokémon a while to evolve and calls out his only Octillery. Marcellus places Octillery in one of the tanks, but the Remoraid quickly swim off in another direction. After being struck by several Water Guns, Octillery retreats into a ceramic jar to console itself. The group are concerned and realize that the Remoraid probably don't recognize Octillery as their friend because they never saw it evolve. After a few ideas, they dress up Ash in an Octillery costume, much to Ash's chagrin. Ash tries to lure out the Octillery, but instead he receives a Water Gun for his efforts.
Overnight, Team Rocket slips into Marcellus's home through a window. Jessie is initially startled by the school of Remoraid because they remind her of her first grade teacher, but James helps her settle down. Meowth suggests that they could open a fishing park and make money for people to catch the Remoraid. With that, the trio commences their plan and vacuums up all the Remoraid. Meanwhile, the group and Marcellus are having dinner when they hear a funny noise. Marcellus decides to check it out. He flips on the light switch and he is greeted by Team Rocket who perform their motto. Marcellus demands to know what they want, then he turns to see that his tanks are empty. Ash and his friends peer through the doorway just as Team Rocket leave through a window. The group and Marcellus rush outside to see Team Rocket leave in their truck. Before the trio drives off, Octillery walks over and clings to the truck's bottom. Octillery leaves a trail of ink behind for Marcellus to follow. Team Rocket stops at a port to load their stolen goods onto a waiting boat. Meowth takes the controls of the crane, but he is stopped by Octillery who suctions itself to his face. Jessie and James attempt to pull Octillery off, but they get headbutted by Octillery's hard head. Octillery soon breaks open the tank containing all of Marcellus’s Remoraid and frees them into the nearby ocean. Now the group has arrived on the scene, but Team Rocket isn't giving up. Meowth tries to grab Octillery with the crane, making a few unsuccessful moves but finally getting it. Ash is about to have Pikachu use Thunderbolt, but Meowth laughs and warns that octillery will be hit as well. With no other option, the Remoraid all fire at the crane, causing Octillery to go flying. But the Remoraid save it with their Water Gun jets, placing it gently onto the port. Marcellus is thrilled to see that his Remoraid have finally recognized Octillery as their friend. Pikachu then unleashes a Thunderbolt followed by Octillery shooting an Octazooka, blowing Team Rocket sky high. After the battle, the Remoraid soon evolve into Octillery themselves and it looks like Marcellus's Octillery won't be lonely anymore.
The next day, the finalist Trainers await at a cliff to watch the Whirl Cup Stadium, partially flooded, rise from the swirling seas. It looks like the real battle just begun.