Arriving at the seaside Ambrette Town while traveling to Cyllage City, our heroes are basking in the wonderful ocean breezes. Ash wants to train there with Froakie, but Serena has another idea: pay a visit to Ambrette Aquarium! Everyone agrees, and they marvel at the vast number of Water-type Pokémon there. They also meet a gentleman who tells them a little about the aquarium and shows them a statue of a golden Magikarp. He also directs them to the aquarium’s curator, Rodman, who’s fishing on the beach with his rather prickly Clauncher.
Rodman tells our heroes a local legend about a golden Magikarp who once rescued a diver in danger, and says he’s trying to catch it for the aquarium so everyone can see it! Ash, Serena, and Clemont want to help him catch it, and he happily hands out fishing rods. Of course, Team Rocket has been listening in on the conversation, and the villains want to catch the golden Magikarp for themselves...
No one is having any luck, so Clemont produces an invention to lure Magikarp in toward the shore. It’s not quite as fine-tuned as Clemont hoped, instead attracting several cranky Sharpedo. Not to be defeated, Clemont reveals yet another invention, determined that this next one will work right. And sure enough, all the fishing rods get a bite at the same time, and with a mighty effort, our heroes and Rodman reel in their catch—and it’s a golden Magikarp! Unfortunately, this one is Team Rocket’s new submarine, which they designed to fool the real golden Magikarp.
After they send Jessie, James, and Meowth blasting off again, our heroes and Rodman are ready to call it a day. But just as they’re about to leave, a magnificent golden Magikarp leaps out of the water! Rodman is delighted to learn that the old story was really true, and promises our heroes that the next time they visit his aquarium, the golden Magikarp will be part of the exhibit!
Ash and his friends arrive at Ambrette Town and are met with a view of the sea. As Bonnie and some of the group's Pokémon play in the ocean, Serena suggests the group visit the Ambrette Aquarium. There, they are told they can see many Water-type Pokémon from different regions, and Bonnie asks the aquarium receptionist to marry her brother. The group looks through the exhibits inside the aquarium, and while they look at the Magikarp exhibit, an old man tells them interesting facts about Water Pokémon. He then takes them to see the statue of the Golden Magikarp. When Ash asks about it, the man tells them to go to the director of the aquarium, who is sitting on the beach, for more information about the statue. Team Rocket, disguised as tourists, watches as the group goes to the beach, and they decide to find out more about the Golden Magikarp as well.
Down on the beach, the aquarium curator introduces himself as Rodman and his partner, Clauncher. He says he is trying to fish for the real Golden Magikarp. He tells the group of an ancient legend. At a time when deep diving was popular, a fisherman was electrocuted by a Starmie, but was saved from drowning by a giant, golden, Magikarp. Since then, there have been occasional reports of the Magikarp being seen again. The curator's dream is to capture the Golden Magikarp so that it can be viewed in the aquarium. Team Rocket, back in their uniforms, watch and listen from underwater in a Magikarp submarine, and decide to get the Golden Magikarp for themselves. Up on shore, Rodman releases his latest catch, a normal Magikarp back into the ocean, and the travelers offer to help him. He gives Ash, Serena, and Clemont fishing rods, and Ash offers to teach Serena how to fish. Later, Serena reels in a Corsola, but fails to capture it. Bonnie and Dedenne attempt to play with Clauncher, but soon it and Dedenne begin to fight, and Rodman tells them that it is not very friendly.
Team Rocket has completely built their new Golden Magikarp submarine, and hope to trick the real thing into coming closer so that they can grab it with the submarine's claws. On the beach, Clemont works on a machine that sends out special sound-waves that attract Magikarp so that the fishers can catch them, however, it only manages to attract Sharpedo, which wreck the machine. Underwater in the golden submarine, Team Rocket attempts to catch a Starmie, which promptly electrocutes them. While nothing bites, Clemont unveils another improved machine for the same purpose as before, but it seems to have no reaction. Everyone feels a pull, however, as Team Rocket's submarine gets caught by their lines and surfaces. The group rejoices, thinking they have found the Golden Magikarp, but soon Team Rocket emerges with their motto and decides to capture the group's Pokémon. The Pokémon at first manage to evade the submarine's claws, but Dedenne is grabbed. Before Ash can send Pikachu, Clauncher begins to attack the claws and rescues Dedenne. Team Rocket calls out Pumpkaboo and Inkay, but Clauncher dodges both of their attacks and Rodman commands it to use Water Gun, Crabhammer and Bubble Beam in quick succession. Ash has Pikachu finish Team Rocket with Thunderbolt.
When the sun sets, the fishers decide to call it a day, and Rodman lets the travelers keep the fishing rods. As they thank him, the real Golden Magikarp leaps from the water. Rodman is thrilled to have finally caught sight of it and promises to have captured the Golden Magikarp by the next time the group visit, and waves them off.