Ash and company are sailing toward Dewford Town, thanks to their new friends Mr. Briney and his Wingull, Peeko. Mr. Briney stops at a small island so that Max and May can enjoy a swim while everyone else and their Pokémon relaxes and soaks up the sun. All the while, Team Rocket is tailing the happy sea goers underwater in their Magikarp sub. Team Rocket is not the only thing following our heroes, though. A dark shadow is following the boat as it turns towards the island. When they get closer to the island, May and Max dive in the water and begin swimming and splashing around. Suddenly Ash and Brock hear May and Max scream. The boat is surrounded by a group of Sharpedo! Ash and Mr. Briney send Peeko and Taillow to help Max and May. Both Max and May were hurt by the rough skin of the Sharpedo. Pikachu uses its Thunderbolt attack to ward off the Sharpedo long enough for Mr. Briney to dock the boat on the island.
While Ash and company set up camp for the night, Team Rocket surfaces, complaining of hunger. The Sharpedo begin attacking their sub and Jessie sends Seviper after them. After one successful strike, the Sharpedo retaliate and send Team Rocket blasting away. The following morning, Ash tries to lure the group of Sharpedo away from the island, but the leader is not tricked. Brock challenges the leader using his Lotad and wins after just two attacks. Brock realizes the Sharpedo is poisoned and begins nursing it back to health, only to discover that it was Jessie's Seviper that poisoned it. Team Rocket attempts to steal Pikachu again, but is warded off by the gang's new found group of Sharpedo friends. The episode ends with everyone swimming happily with the group of Sharpedo, which, they discover, only have rough skin when threatened or are around enemies. Now it's off to Dewford to compete for a second Hoenn badge!
Aboard the S.S. Wingull, Ash and his friends admire the calm sea and the salty air. Mr. Briney remarks the conditions are wonderful for sailing, happily declaring that they'll reach Dewford Island before nightfall. May calls out her Pokémon to enjoy the boat ride, with Ash and Brock following suit. Team Rocket are in their Magikarp submarine, tailing the group’s boat. Jessie orders her teammates to start pedalling harder as the S.S. Wingull speeds ahead. Seeing all the Pokémon relaxing, Max and May decide they want to go swimming. Ash wants to go straight to the Gym. However, the others are able to convince him that one swim won't stop him from becoming a Pokémon Master, so they change course and head for an island shaped like a crescent moon. As they sail on, a shape under the water is following them, and it's not Team Rocket. Mr. Briney tells them that he discovered the island they're going to, and it's not on any map. May comes out in a new swimsuit that she bought in Rustboro City, and she and Max jump into the water. Ash, Brock, and all the Pokémon except Torchic prepare to dive in as well, but they hear screaming. May and Max are surrounded by Sharpedo. Mr. Briney has Peeko carry them back to the boat, while Pikachu and Taillow try to attack the Sharpedo. One jumps up and rubs against Max's leg, and its Rough Skin scratches him. Peeko deposits them back on the boat, and May is very angry when she realizes that her brand new bathing suit was torn. While Brock checks on Max’s injuries, Ash takes the opportunity to check Sharpedo out on his Pokédex. Mr. Briney tells them that they should leave, but then the Sharpedo start ramming the boat. With a little help from Pikachu's Thunderbolt, they're able to get passed the Sharpedo and dock on the island.
That night they're all sitting around a campfire eating dinner, while some lookouts from the Sharpedo gang are patrolling the waters. Brock and Mr. Briney don't understand why the Sharpedo are working together since they're usually solitary, but the more important question is how they are going to get off the island. Ash thinks that a Pokémon battle would be all it would take, but Max reminds him that they only have eight Pokémon in total while there are lots of Sharpedo. Brock thinks that if they beat the leader of the Sharpedo in a battle, they might become the leaders. Since they don't have any other plans, they decide to try it. Meanwhile, Team Rocket surfaces, exhausted. Jessie notices the island in the distance, but before they can do anything about it they're surrounded by the Sharpedo. Jessie sends out her Seviper and it lands a Poison Tail on the leader. However, all of the Sharpedo charge forward which sends the Rocket trio blasting off. As they're flying off into the distance, they see Mr. Briney's boat docked on the island and realize the twerps are there.
It's now morning, and the entire bay is full of Sharpedo. Ash and Pikachu stand in a large bucket and hold on to a rope attached to Taillow and Peeko. This functions as a water ski of sorts, and Ash sails around the bay in an attempt to get the Sharpedo to follow. The leader orders the rest of the school to pursue Ash, and Ash soon leads them out to sea. Then May, Max, and Mr. Briney pull a rope they had hidden, and a wooden fence slides into the water, trapping the Sharpedo leader. It rams the fence over and over, pulling on the rope, until it's yanked from Max and Mr. Briney's hands. May keeps holding on so she flies forward and lands in the water, where the Sharpedo charges at her. Brock quickly sends out his Lotad, which distracts the Sharpedo. In the meantime Max has May grab a hold of the rope as he and Mr. Briney pull her to shore, as well as completing their fence trap. As Ash returns too, Brock challenges Sharpedo to a one-on-one battle. Brock orders a Water Gun, but Sharpedo snaps Lotad up in its jaws. He calls for another Water Gun, and Lotad follows through and manages to free itself. Brocks orders yet another Water Gun, but Lotad swims up to Sharpedo and stares at it. Suddenly Sharpedo keels over, and everyone realizes that something is amiss. Brock examines Sharpedo close to the shore and declares that it must be poisoned. Max suggests Sharpedo could have gotten into a fight with a Tentacruel. Brock does everything he can for Sharpedo and feeds it some Pokémon food to keep its strength up, but they don't have any medicine. Brock continues to offer Sharpedo comfort, urging it to overcome the poison’s effects. Sharpedo wakes up early the next morning and sees that Brock fell asleep petting it, and it gently puts its fin over him before going back to sleep.
Suddenly the Magikarp submarine surfaces, waking everyone up. It shoots out nets and grabs Pikachu and Sharpedo. Brock demands Sharpedo be returned, admitting that it was poisoned. Jessie smirks as she declares that her Seviper's Poison Tail worked, though the admission irritates Brock. He has Lotad use Razor Leaf to cut the nets. Ash tells Pikachu to use Thunder, but stops mid-sentence and says that they don't even need to bother. Team Rocket is confused until they look behind them and see the entire school of Sharpedo. They all use advance towards Team Rocket at the same time causing the submarine explodes and sending Team Rocket blasting off again. Sharpedo has now recovered from the poison, and it's grateful for everything Brock did. All the other Sharpedo start nuzzling Brock and he freaks, but then he realizes that their Rough Skin isn't affecting him. Mr. Briney explains that many Pokemon traits stop working if they have befriended someone. The others try petting the Sharpedo and they aren't hurt either. The group arrives at Dewford Town by the evening. Mr. Briney wishes them all the best as he sets course for Slateport City, and they wave goodbye to both him and the school of Sharpedo.