Everyone is relaxing on the beach as the episode opens. Ash is taking care of his Poké Balls and May is worrying about why her Wurmple isn't eating. Corphish, Ash's new Pokémon, is nibbling on some seaweed and starts to eat some rope before Ash can stop it. Team Rocket appears and snatches Pikachu. Wurmple uses String Shot to try and stop their balloon while Ash and company pull on the string to stop Team Rocket's escape. When Corphish tries to help, though, it snaps the string with its claws.
Ash and the gang hunt down the balloon and a fight ensues between the two groups to regain Pikachu. After Pikachu is back in Ash's arms, May and Jessie begin to argue (again) about whose Wurmple is better. To everyone's surprise, both Wurmple evolve. May's evolves into a Silcoon and Jessie's evolves into a Cascoon. Jessie thinks that hers is also a Silcoon, though. After another failed attempt at trying to catch Pikachu, Team Rocket is sent blasting off again. Corphish faints at this point and Ash realizes he has to rush it to the nearest Pokémon Center. Along the way, he must cross a large river guarded by a school of aggressive Carvanha. Seeing no other way to cross and with Corphish's condition worsening, Ash decides to wade across the river with his Pokémon just barely keeping the Carvanha at bay.
While all this is going on, Max, May, Brock, and the rest of their Pokémon hear an Aron down the beach. They go to inspect the noise and end up trapped in a cage by Team Rocket. Team Rocket only has a moment to be upset about Pikachu not being in the cage before they fall into their own trap. Taillow, the only one not to get trapped, takes a tape-recorded message to Ash. Ash, with Nurse Joy's help, hurries back just in time to send Corphish to free his friends before the tide comes in and immerses the cages in water. Team Rocket fails again at catching Pikachu and everyone happily thanks Corphish for its help.
The episode starts with Ash and his friends camping out near a lake, where Ash is cleaning his Poké Balls in preparation for his rematch with Brawly. May comments that Ash is really working hard, and Max then comments that she should follow Ash's example. After Ash finishes speaking to his Pokémon, Corphish wanders away after it spots a piece of seaweed. It begins to eat the long piece of seaweed-- which was tangled up with some buoys attaches to a rope-- and ends up eating the buoys as well. Ash tells it to stop, warning that it was going to get sick. Corphish then pulls on the rope, causing it to snap. Its claw goes flying backwards and then smacks Ash. Later, Ash introduces Corphish to all of the Pokémon, and Corphish greets them back by smacking them. May grabs her Pokémon and yells at it, warning that it would never make friends that way. Brock disagrees, saying that it might be how Corphish say "hello".
Later during breakfast, May notices that her weary-looking Wurmple is not eating, and questions if it is sick. Torchic offers its own food to it, but it still refuses. Corphish then appears out of nowhere, and eats both Wurmple and Torchic's food before all of a sudden, a robotic hand comes from above and grabs Pikachu. Team Rocket then start their motto, wearing Corphish claws. They put Pikachu inside an electric-proof cage, rendering its electric attacks useless. May then commands her Wurmple to use String Shot on Team Rocket's balloon, and all four Trainers grab on to the string of silk, trying to hold Team Rocket back. Ash then makes the mistake of telling Corphish to help. When it grabs the string, its claws snap it, causing Team Rocket's balloon to go flying out of control and crash. May starts to yell at Corphish, but Max says that it was Ash's fault for telling it to help. They then pursue Team Rocket.
It doesn't take them long to find them. Ash then commands Corphish to attack with BubbleBeam, which causes Pikachu's cage to go flying. James attempts to stop them with Cacnea, but it turns around and hugs him. Jessie then calls out Seviper, who caught Pikachu's cage with its tail. May then tells Wurmple to use String Shot to grab Pikachu's cage, and it works, resulting in Pikachu returning in Ash's arms. May thanks Wurmple, praising that it was the best. Jessie then feels the need to show off her Wurmple, as she thought hers was better. Brock then comments that all Wurmple looks the same to him. May and Jessie began to argue with each other over whose Wurmple is cuter before Jessie's Wurmple starts to glow white. A few seconds later, May's Wurmple also starts to glow white. Both Wurmple then evolve into Silcoon, or so Jessie thought. When Jessie picks it up, James and Meowth notice that it looks different from May's Silcoon, but they ignore it. They then attack Ash again, but he counters with Corphish's Crabhammer, which sends Cacnea flying backwards. Ash then commands Pikachu to use Thunderbolt, which usually sends Team Rocket blasting off.
As they begin to celebrate their victory, Corphish's skin turns bright red, and it falls over. Brock checks its temperature, and discovers it has a fever. Ash reaches for its Poké Ball, but then realizes he left them back at camp. Convinced there was no time to go back, Ash decides to carry Corphish to the Pokémon Center. He takes Pikachu with him while leaving Treecko and Taillow behind. As Ash begins his hike up the mountain, he notices Corphish becoming hotter, and stops to think how he could bring its fever down, before he then spots water trickling down the mountain side. He scoops some of it up with two big leaves and gives it to Corphish, which slightly helps it. Ash then hears his stomach grumbling, realizing he never got to eat breakfast due to Team Rocket. Pikachu then spots a Pecha Berry tree, which Ash picks some Berries off of. As he begins to eat one, he realizes that he has stepped on a Shroomish. He apologizes to it, but the Shroomish does not accept the apology, and attacks Ash with Stun Spore. Ash then runs away with his Pokémon as fast as he can.
Later, Ash begins to see Dewford Town in the distance. All he has to do now is cross a river. As he begins to cross it, a school of Carvanha then attack, and scare him out of the water. As Ash formulates a plan on how he could cross the river, he spots a rowboat near the shore. The Carvanha start to attack again, and Ash commands Pikachu to hold them off with Thunderbolt. It works for a little bit, but they then come back and eat the boat. Ash runs back to shore, and formulates another plan to get across the river and quick, as Corphish's fever is steadily increasing. A little bit downstream, Ash spots a bridge. Thinking their worries are over, Ash is disappointed to see that the bridge is destroyed, but decides to just cross the river by wading through it. Ash then puts Pikachu and Corphish on a wooden plank and carries them over his head. The Carvanha soon spot him, and begin attacking again. Ash then starts to move quicker as the shore is just feet away, and commands Pikachu to jump to shore, and to attack Carvanha with Thunderbolt. Pikachu does its best to keep them at bay, but a couple of them manage to attack Ash. Corphish starts to worry about Ash, but he promises Corphish that he wouldn't let anything happen to it. One Carvanha attacks Pikachu, making it unable to stop any further attacks. As a group of Carvanha are about to pounce on Ash, Corphish fires a BubbleBeam attack at them, which stops them long enough for Ash to get to shore. Relived, Ash takes a small break, and thanks his Pokémon before going to the Pokémon Center. Around sunset, Nurse Joy reveals to Ash that Corphish should be just find in the morning, and it seems that the buoys that Corphish ate earlier is what made it sick. Nurse Joy questions why Corphish ate them, but Ash is unable come up with an answer.
Later, Brock, May, and Max are seen around a campfire when all of a sudden, they start to hear what Brock believes to be an Aron. Max is puzzled, as Aron usually stay in caves and never came out on the beach, and they soon came across a tape recorder with Aron's voice playing on it. Before they could question it, they all-- except for Taillow-- suddenly find themselves trapped in cages, and they soon find out that it was Team Rocket who set the trap. They then also discover that Pikachu wasn't with them, and James realizes that only two of the traps were activated as he accidentally activates the third, which traps Team Rocket as well. Treecko attempts to break the cage with Pound, but to no avail. Little did they notice, the tide starts to come in. Back at the Pokémon Center, Ash is at Corphish's bedside, dreaming of himself battling using it. He commands Corphish to use Crabhammer, and it then lightly bonks him on the head. Back at the beach, Brock finally realizes that the tide is coming in, and that they were all at risk of drowning. Brock then looks at Taillow, and gets an idea.
The next morning, Corphish feels 100% better, as noticed by Ash. Taillow then comes flying in from the window with a tape recorder around its neck, and Ash plays it hearing a recording of Brock talking about the situation and telling Ash to hurry. Back at the beach, the tide is already up to their necks. They then spot Corphish swimming at full speed towards them, with Ash and Nurse Joy in a speed boat following it. Ash then orders Corphish to use Crabhammer on the cages, which immediately sets everyone free. Back at the dock, Team Rocket thanks Ash for saving their lives, but it does not stop them from making one last attempt to steal Pikachu. Ash then commands Pikachu to use Thunder on them, followed by Corphish's BubbleBeam, which blasts them off for the second time. Afterward, Brock thanks and flirts with Nurse Joy at the same time, and May and Max then thank Corphish, which then happily smacks everyone around.