Ash, Misty, and Tracey have stopped on an island to rest. They've sent a few of their Pokémon out to stretch, and a few ride Lapras around. One of these is Misty's Psyduck who, asleep, falls off. Lapras swims back to shore, and Pikachu and Togepi tell Ash and his friends that he's missing. Misty is horrified and they send out the rest of their water Pokémon to help look. Nearby, a girl is fishing while her Tentacruel swims about. Psyduck surfaces, flailing in the water.
The Pokémon return, but with no luck. Misty is still upset, and Ash notes how unusual this is. Tracey voices his opinion on the subject, saying that he believes Psyduck is Misty's favorite Pokémon and that she just hides her feelings. The girl with the Tentacruel rows over and asks them if the Psyduck she found is theirs. It is, but its tail is glowing a bright pink.
Misty thanks the girl and she 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 their favorite Pokémon. They talk about water Pokémon for a moment, completely shutting out Ash and Tracey. After showing each other some lures they made and wishing they could catch one of the legendary Pokémon of the sea (the Dragon Type family), Marina asks Misty why Psyduck can't swim. Misty covers for it, saying that its a rare breed. Marina then suggests that they battle each other to see which is the most powerful water trainer. Misty agrees.
Meanwhile, Team Rocket peers at the shore through the periscope of the Magikarp sub. Meowth yells at Jessie for losing Ash and his friends, only to be whacked by her huge 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 them. They sit and watch, waiting for the right time to strike.
As Team Rocket watches, Misty and Marina's battle begins. They agree to a 3 on 3 battle, and send out their first Pokémon. Marina uses Tentacruel and Misty sends out Goldeen. Goldeen's Agility lets it dodge Tentacruel's Wrap, and it lunges for a Horn Attack. Unfortunately, Tentacruel stabs it with a Tentacruel, and it falls to the surface, fainted. 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, and Marina compliments her on how good it looks. It as powerful as it is good-looking and it 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 so the water hits Staryu. Misty tosses out a Pokéball to use her Psyduck, but it falls into the water. It was empty. Embarrassed, Misty tells Ash to get Psyduck's Pokéball for her, but he can't tell which one it is. He tosses her her bag, but it falls short. She reaches into the water to get it, and when she pulls it up, a Pokémon's head is stuck in it. She yanks it off, and everyone is amazed: it's a Golduck! Apparently, Psyduck evolved.
Misty runs over and hugs it. She says that nobody will yell at it like they did to Psyduck. Ash comments to Tracey that the person who yelled at it the most was Misty herself, and he tells Ash that it only supports his theory. He thinks that Psyduck evolved because it loved her, too. 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, makes it go back at Marina. Just as it is about to break, the wave twists into an inverted whirlpool. Golduck jumps off just in time to avoid the Magikarp sub, which pops out of the water.
Everybody hurries away, and Pikachu's attempts to shock the sub fail. A wave from the sub sweeps over them, and a net is fired. Team Rocket climbs out and discovers that they didn't capture Golduck and Pikachu as they'd hoped, but have netted Marina's Tentacruel and Psyduck. Ash and his friends cling to the tree the wave swept them onto, and Marina realizes that her Pokémon are missing. They jump down and she sends out a Starmie who swims over to the sub. Golduck jumps in too, and quickly outdistances Marina's Pokémon.
Team Rocket argues amongst themselves about whose idea it was to attack, and Golduck jumps out of the water. Team Rocket moves to attack them, but they are thrown onto the beach by Golduck's Psychic abilities. Team Rocket crawls out of the sub and say their motto (it's about time!). Ash tells them to give Marina back her Pokémon and Jessie and James seem all to happy to oblige. Before Marina can get any closer, however, Meowth jumps up and insists that Ash hand over Pikachu before Marina gets her Pokémon back.
Marina tells her Pokémon to wiggle out of the net, but they are too tired. She runs at Jessie, but she stops her with Arbok. It lunges at her, but is slammed into a tree by a stream of water. Everyone looks over and sees Golduck walking up from the water. It blocks Arbok's Poison Sting with a Barrier, then slams it into the air with a Psywave. Team Rocket runs back into their sub, but Golduck destroys it with a Hyper Beam. Team Rocket is thrown into the air and out of the episode. As the sub burns, Marina yells out for Tentacruel and Psyduck. Golduck walks out, carrying both above its head and protecting them with another Barrier. Marina thanks it and Misty hugs it.
Moments later, Misty and Marina are back at the rocks from before, continuing their battle. Marina uses Starmie, who uses Tackle against Golduck. It then hits hit with Bubble. Golduck sends out a Hyper Beam, and Starmie creates an Ice Screen to block it. It almost works, but it shatters and the Beam connects with its target. Misty wins and she cheers her Golduck. Misty holds out a Pokéball and tells Golduck to take a rest, but Psyduck pops out of it. Everyone is stunned. As Misty tries to sort out the situation, Golduck notices three girls walking along the beach. It runs over and starts flexing its muscles. Marina realizes that it wasn't Misty's Golduck at all, it was just a Golduck that liked to show off in front of girls. "It's just a Pokémon version of Brock," agrees Misty, shattered.
Later, Ash asks where Psyduck was all this time. It was resting in its Pokéball. Misty asks if it is going to evolve into Golduck anyway, but its tail isn't glowing anymore. Marina congratulates her, however, saying that she was able to control a wild, evolved Pokémon without catching it. She is cheered up until Ash notes that since Golduck wasn't hers, the battle with it didn't count.
That evening, they all say farewell to Marina. Misty tells her that she wants a rematch once Psyduck finally does evolve and Lapras rides off into the sunset.