Upon arriving at Kinnow Island, Ash and his friends decide to rest and send out a few of their Pokémon to stretch. Eventually, Misty's Psyduck falls off Lapras while falling asleep. Upon being informed of Psyduck's disappearance by Pikachu and Togepi, the group sends out the rest of their Water-type Pokémon to look for it, but to no avail. Misty is left upset by this, and Ash notes how unusual this is. Tracey says that he believes Psyduck is Misty's favorite Pokémon and that she is just hiding her feelings for it. Just then, a girl arrives and asks them if the Psyduck she found is theirs. It is, but Psyduck's tail is glowing a bright pink.
Misty thanks the girl, who introduces herself as Marina. She points at Psyduck's glowing tail, and says that a Trainer once told her that when this happens, the Psyduck is about to evolve. Ash checks his Pokédex and it tells him that Golduck, Psyduck's evolved form, is the fastest of all Water Pokémon. Both Misty and Marina say that Golduck is one of their favorite Pokémon. They talk about Water Pokémon for a moment, completely ostracizing Ash and Tracey. After showing each other some lures they made and wishing they could catch one of the legendary Pokémon of the sea, Marina asks Misty why Psyduck cannot swim, since she found it flailing in the water. Misty claims that it was because it was a rare breed. Marina then suggests that they battle each other to see which is the most powerful Water Pokémon Trainer, and Misty agrees.
Meanwhile, Team Rocket peers at the shore through the periscope of the Magikarp submarine. Meowth yells at Jessie for losing Ash and his friends, only to be whacked by her long hair. James asks to find a place for lunch, and Jessie tells them all to stop whining. She looks through the periscope once more and notices Misty and Marina getting ready to battle. They sit and watch, waiting for the right moment to strike.
As Team Rocket watches, the battle between Misty and Marina begins. They agree to a three-on-three battle, and send out their first Pokémon. Marina uses her Tentacruel and Misty sends out her Goldeen. Using Agility, Goldeen dodges Tentacruel's Wrap and lunges for a Horn Attack. However, Tentacruel stabs it with Poison Sting, defeating it. Marina tells Misty that she likes Water Pokémon that have another type as well, as this gives her a slight advantage. Misty sends out Staryu, which defeats Tentacruel with a single Swift attack.
Marina sends out a Psyduck of her own, who looks a lot stronger than Misty's. Staryu uses Water Gun, but Psyduck deflects it with Confusion and it hits Staryu, defeating it. Marina suggests that Misty send out her Psyduck, and Tracey tells Misty that a battle might be enough to make Psyduck evolve. Misty tosses a Poké Ball, believing it to be Psyduck's, but it falls into the water empty. Embarrassed, Misty tells Ash to get Psyduck's Poké Ball for her, but he is unable to tell which one it is. He tosses her backpack to her, but it falls into the water. She reaches into the water to get it, and when she pulls it up, the group finds a Golduck's head stuck in it.
Misty runs over to Golduck and hugs it, believing Psyduck evolved. Tracey believes that Psyduck evolved because it loves her. The battle continues, with Misty using Golduck. Psyduck uses Confusion, and a wave races at Misty. Golduck leaps onto the wave and, using its Psychic powers, deflects it back to Marina. Just as it is about to hit, the wave twists into an inverted whirlpool. Golduck jumps off just in time to avoid Team Rocket's Magikarp submarine, which pops out of the water.
As everyone flees from the wave, the submarine fires a net, capturing Marina's Tentacruel and Psyduck. Marina sends out a Starmie, who swims over to the sub, joined by Golduck. Golduck attacks Team Rocket, throwing the submarine to the beach using its Psychic abilities. Team Rocket crawls out of the sub and recites their motto. When Ash tells them to give Marina back her Pokémon, Meowth insists that he hand over Pikachu first.
Marina tells her Pokémon to wiggle out of the net, but they are too tired. She runs at Jessie, but Jessie sends out Arbok. It lunges at her, but Golduck protects Marina by slamming it into a tree with a stream of water. It then blocks Arbok's Poison Sting with Disable, then slams it into the air with Psychic. Team Rocket runs back into their submarine, but Golduck destroys it with a Hyper Beam, sending them blasting off again. Meanwhile, Golduck rescues Tentacruel and Psyduck from the burning submarine.
Moments later, Misty and Marina resume their battle. Marina uses Starmie, who uses Tackle and then Bubble against Golduck. Golduck responds with Hyper Beam, and despite Starmie's attempt to defend itself with Light Screen, the attack defeats it. Misty then holds out a Poké Ball and tells Golduck to take a rest, but Psyduck pops out of it, stunning everyone. As Misty tries to sort out the situation, Golduck notices three girls walking along the beach, runs over, and starts flexing its muscles for them. Marina realizes that it wasn't Misty's Golduck at all, but just a Golduck that liked to show off in front of girls.
Later, Ash asks where Psyduck was all this time. Misty asks if it is going to evolve into Golduck anyway, but finds that its tail isn't glowing anymore. Marina congratulates her, however, saying that she was able to control a wild, evolved Pokémon without catching it. Misty cheers up until Ash says that since Golduck wasn't hers, the battle with it didn't count. That evening, as the group bids farewell to Marina, Misty tells her that she wants a rematch once Psyduck finally evolves.