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.
Ash and his friends are camping on Dewford Island as Ash continues to train for his rematch against Brawly. As Ash polishes up his Poké Balls, Max suggests that May follow Ash's example, but she quickly puts her younger brother in his place with a threat. 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 is going to get sick. Corphish then pulls on the rope, making it snap which causes its claw to fly backwards and smacking Ash away. Later, Ash introduces Corphish to all of the group's Pokémon, and Corphish greets them back by smacking them. May grabs Torchic and warns Corphish that it will never make friends that way. Brock disagrees, saying that it might be how Corphish say "hello".
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 scoffs down both Wurmple and Torchic's food before all of a sudden, a robotic hand appears from above and grabs Pikachu. Team Rocket then recites their motto while side stepping and wearing Corphish claws. They put Pikachu inside an electric-proof cage, rendering his 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, however, makes the mistake of asking 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. Ash is unable to say anything, so he orders everyone to pursue Team Rocket.
It doesn't take the group long to find the Rocket trio. Ash commands Corphish to attack with Bubble Beam, which causes Pikachu's cage to go flying. James attempts to stop them with Cacnea, but it turns around and hugs him instead. Jessie calls out Seviper, who catches Pikachu's cage with its tail. May orders Wurmple to use String Shot to grab Pikachu's cage, and it works, resulting in Pikachu returning to Ash's arms. May thanks Wurmple, praising that it was the best. Jessie is immediately annoyed, and she feels the need to show off her Wurmple, as she thought hers was better. Brock then comments that all Wurmple look 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. May's Wurmple also starts to glow. Both Wurmple suddenly 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 decide to attack Ash again, but he counters with Corphish's Crabhammer, which sends Cacnea flying backwards. After that, Pikachu uses Thunderbolt, sending Team Rocket blasting off.
As they begin to celebrate their victory, Corphish's skin turns bright red, and it collapses. 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 is 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 a mountain, he notices Corphish becoming hotter. He pauses for a moment to think how to bring its fever down, before spotting 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 soon spots a Pecha Berry tree, which Ash picks some Berries. As he begins to eat one, he realizes that he has stepped on a Shroomish. He quickly apologizes to it, but the Shroomish is too aggravated and uses Stun Spore, forcing Ash, Pikachu and Corphish to flee the area.
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 attacks, and scares him out of the water. As Ash formulates a plan to 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 the Carvanha return with a vengeance and begin gnawing 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. Further downstream, Ash spots a bridge. Thinking their worries are over, Ash is disappointed to see that the bridge is destroyed, so he decides to just cross the river by wading through it. Ash carries Pikachu and Corphish on a wooden plank with them over his head. The Carvanha soon spot him, and begin swarming again. Ash starts to move quicker as the shore is just several feet away, and commands Pikachu to jump to shore, and to attack Carvanha with Thunderbolt. Pikachu does his best to keep them at bay, but more Carvanha appear in the opposite direction and 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 knocks Pikachu to the ground, allowing the Carvanha to continue their frenzied attack on Ash. As a group of Carvanha is about to pounce on Ash, Corphish fires a Bubble Beam 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 fine in the morning, and it seems that the buoys and rope 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, as he doesn't know.
Overnight, Brock, May, and Max are sitting around the 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 venture out onto the beach. The trio leave the camp to investigate and they soon come across a tape recorder with Aron's voice playing on it. Before they can question it, they all, except for Taillow, suddenly find themselves trapped in cages. Team Rocket emerge from hiding to check out their catch, but their hopes are dashed upon learning that Pikachu isn’t with them. James also realizes that only two of the traps were activated as he accidentally activates the third, which traps himself and his teammates. 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 using it in a battle. 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 the tape recorder, and gets an idea.
The next morning, Corphish feels much better, as noticed by Ash. Taillow then comes flying in through a window with the tape recorder around its neck. Ash presses play on the tape recorder and learns that Brock and the others have been trapped in cages because of Team Rocket. Back at the beach, the tide is already up to their necks. Corphish leads the rescue crew, with Nurse Joy driving Ash aboard her speedboat onto the scene. Ash has Corphish 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 is unimpressed and he commands Pikachu to use Thunder on them, followed by Corphish's Bubble Beam, which sends Team Rocket blasting off. Afterwards, Brock thanks and flirts with Nurse Joy, while May and Max thank Corphish, which then happily smacks everyone around. As everyone rubs their bruises, May suggests Corphish try a less-is-more approach when it comes to its gratitude.