Ash and his friends are on their way back to New Bark Town via a rocky road. The group comes across a tunnel, which Brock suggests that it may lead to the ocean. Misty perks up and promptly runs through, and as predicted the group finds themselves staring at the sea. Misty takes a deep breath of the salty air before calling out her Water Pokémon. Staryu, Corsola, Goldeen and Politoed enjoy a break in the ocean water. Psyduck wanders towards the water, but a slight splash is enough to terrify it to the verge of tears, and Misty gently reminds it that it can't swim.
They continue along the beach, and they see a sign for a synchronized swimming event. Brock becomes very excited, imagining pretty girls in bathing suits performing alongside Pokémon. He promptly takes off, forcing Ash and Misty to run after him. He runs up to a boy on the beach and asks about the event, and the boy informs him that it's just about to start. Ash and his friends head into a huge stadium and sit down in the stands, and Brock becomes even more excited when Ash comments on whether it will be like the Cerulean Gym, and lavishes praise on Daisy, Lily, and Violet. Misty is none too pleased and urges Brock to settle down. As the show starts, a Mantine, Qwilfish and Lanturn leap from underwater, followed by a young man, much to Brock's dismay. Ash and Misty watch the show while Brock sobs into his arm, the former two complimenting the performer’s talents with his Pokémon. Meanwhile Team Rocket is standing up above the stadium watching, also impressed by the skill of the Pokémon. Meowth then spins a tale about how impressed Giovanni would be with the Water types to cheer him on in his morning swim. Jessie and James are instantly taken with the Meowth’s vision and set their sights on capturing the Water Pokémon.
Ash and Misty heap praise on the boy after the show, with Misty talking over Brock's requests for pretty girls. The performer introduces himself as Dorian, the Leader of the Coastline Gym. He gives the group a tour, and the trio sees photos of Dorian with Misty's sisters. Dorian explains that he has been to the Cerulean Gym many times and dreams of having a Gym just like it, as a lover of Water Pokémon. Dorian explains that for the time being, Coastline is just a private Gym, which explains to Brock and Ash why they'd not heard of it before. Misty, however, is more eager as to whether her sisters mentioned anything about her to Dorian. Much to Misty's horror, all Dorian had heard was that the girls were sad that the baby of the family was such a loser. Misty bites everyone's heads off and the argument turns to who loves Water Pokémon the most. Dorian disputes Misty's claim, and she challenges him to a battle, seeking to prove herself as the best Trainer from Cerulean.
Later, Team Rocket is in their Magikarp submarine, when they come under attack from Dorian's Pokémon. The submarine springs a leak and begins to sink. Meanwhile, Dorian shows the friends his underwater stadium designed to give Water Pokémon the chance to reach their full potential, which Misty praises, commenting that she's always dreamed of having a real underwater battle. Ash notices an odd-looking Magikarp float past, chased by Dorian's Pokémon, but Misty immediately recognizes it as Team Rocket's submarine. The submarine reaches the surface, and the trio leaps out for air. They find themselves confronted by Ash, who sends Totodile out to blast the sub with Water Gun. Mantine and Lanturn join in with Bubble Beam and Water Gun, respectively, sending Team Rocket blasting off.
Dorian asks Misty if she's ready for the battle, but Ash pleads to be allowed to give Totodile a run before Misty battles. Misty reluctantly agrees, and so Ash and Dorian descend into the underwater stadium, where Brock acts as a referee. Totodile goes up against Lanturn, both managing to dodge each other's Water Gun, leading Misty to note how quickly Totodile moves underwater. Totodile is ordered to use Scratch, but Lanturn uses Supersonic and confuses it, causing it to flip upside down. Lanturn then hits Totodile with Take Down, before knocking it out of the water with a follow-up move. Both airborne, Lanturn flips around in mid-air and uses Take Down again, knocking Totodile back into the water and slamming it against the ocean floor. Misty points out that Ash's comfortable style of battling won't apply to a place where Lanturn can swim above and below his Pokémon. Totodile gets up and uses Slash on Lanturn, striking it hard. However, Lanturn counters with Spark, hitting Totodile and shocking it, before ramming it with Take Down which knocks it out, giving Dorian the match. Brock tells Ash that Dorian was just much more experienced in the Gym’s aquatic setting.
Misty then challenges Dorian to a three-on-three battle. Misty calls on her Goldeen, but Psyduck pops out first. However, upon realizing it is underwater, it frantically scrambles for the surface and reaches the edge of the pool. Misty sympathizes with Psyduck and they get a flotation ring out so it can still enjoy the water. Back underwater, Misty's Goldeen goes up against Qwilfish. Goldeen's Horn Attack is dodged by Qwilfish, who in turn strikes with Tackle. Goldeen uses Agility to dodge Qwilfish's other Tackles, and then lands a Horn Attack on it. Goldeen then uses Supersonic, but Qwilfish shrugs off its effects and fires Pin Missile. Goldeen manages to reflect those coming its way by spinning its tail. Goldeen aims a Horn Drill, which Qwilfish dodges. Qwilfish follows up with a Pin Missile, hammering Goldeen from behind. A final Take Down is enough to knock Goldeen out and win the round for Qwilfish.
Promising something completely different, Misty sends Corsola out to battle Mantine. Misty mutters to herself that she cannot afford to lose. Outside, Team Rocket is clad in scuba gear, preparing to make another attempt. Mantine uses Supersonic on Corsola, but Corsola shrugs it off and rams Mantine with Tackle. Mantine recovers with two Wing Attacks, each striking Corsola hard. Mantine performs Agility before moving in for a Tackle attack, dodging Corsola's Spike Cannon and then ramming it into the rocks. Much to Dorian's surprise, Misty orders Recover, then full strength Corsola uses Spike Cannon again. This strikes Mantine, and a follow-up Tackle is enough to win the round for Corsola and Misty, evening the score. Dorian recalls Mantine and comments that Misty is pretty powerful after all.
In the third round, Dorian's Lanturn steps up, but before Misty can send either Staryu or Politoed, a torrent of water interrupts the battle, knocking everyone over. Team Rocket arrives in a giant crab robot and recites their motto. One of the claws opens and begins to draw water in, sucking the Trainers forward. Totodile and Qwilfish attempt to stop the robot, but Team Rocket amp up the suction power and both Pokémon are captured. Mantine and Lanturn are also helpless to resist. Misty sends out Corsola, outside of the suction, and it Tackles the arm, pointing it skyward. Psyduck and its flotation ring are then sucked underwater. Misty desperately leaps forward and pushes Psyduck aside, but is sucked into the machine herself. Dorian and Corsola rush to her aid, but they are also sucked in. Team Rocket then reverses the claws, and the left claw ejects water, blowing Brock, Ash, and Pikachu away. When the dust clears, the giant robot has vanished, and Ash vows to come and rescue them.
Above onshore, Ash sends Noctowl for an aerial search. It soon spots something, so Ash and Brock run after it. Psyduck, not wanting to be alone, toddles after them. On the surface, Jessie and James gloat over their prize, while Misty and Dorian struggle to free themselves from their bonds. Ash and Brock quickly arrive, led by Noctowl. Team Rocket send out Victreebel and Arbok. Ash is about counter with Phanpy, but Psyduck leaps into the fray first, angry and determined, unlike the usual attitude it displays. As Brock unties Misty and Dorian, Ash wonders if Psyduck wants to battle. After a pause, Psyduck turns back around and looks confused. Furious, Jessie orders Arbok to Bite Psyduck, and it clamps onto its head. Victreebel fires a Razor Leaf, which causes Psyduck to flail around in pain. Misty jumps up and tells Arbok that it's not biting hard enough. With a huge headache, Psyduck uses Confusion, sending Victreebel and Arbok flying back at their Trainers. Psyduck then uses Psychic to send a powerful force of energy at Team Rocket, destroying their robot and sending them blasting off. Dorian is left in awe of Psyduck's surprising strength.
Ash, Misty, and Dorian reunite with their Pokémon, and Dorian comments again on Psyduck, who merely looks confused at the compliment. Misty says that it's a shame that they couldn't finish their battle, but Dorian says that had she used Psyduck, she would have easily won. With the score at 1-1, the two decide to call it a draw. Dorian tells Misty that he thinks she's every bit as good as her sisters, and Ash and Misty admit they've learned a lot from being there. Ash promises to tell everyone about the Coastline Gym when they reach the Silver Conference. Dorian thanks them sincerely, and promises to inform Misty's sisters on how well Misty is doing when he next visits Cerulean City. In a show-off of Dorian's admiration for Misty's strength, a photograph of himself and Misty posing with Corsola and Psyduck is now pinned alongside his pictures of the Sensational Sisters. The group bids farewell to Dorian and continues on their way towards their next adventure.