At the Cerise Laboratory, Goh receives a notification about his next Project Mew Trial Mission. Going through the three options he's presented with, he chooses the one that involves catching a wild Kingdra, given his skills at catching Pokémon. Chrysa shows him and Ash Kingdra's Pokédex entry, which mentions that Kingdra can create whirlpools by yawning, and are known to have fierce battles with Dragonite. Goh momentarily hesitates when Ren tells him that catching a wild Kingdra can be difficult, but Ash manages to reassure him that he can do it. Figuring that Kingdra could show up in the same area as its pre-evolution Seadra, Goh searches for a Seadra habitat on his Rotom Phone and settles on Slateport City in the Hoenn region, where whirlpools sightings have also been made.
Ash and Goh waste no time in traveling to Slateport, where they ride Goh's Dewgong and Mantyke out into the sea while towing a rubber raft that Pikachu and Grookey ride on. Once sufficiently far from the shore, they perform several dives, but fail to spot any Kingdra or Seadra. During one of the dives, Goh spots a school of Luvdisc and attempts to catch one, but his thrown Poké Ball floats to the surface before it can reach its target. Later, Goh spots something at the seafloor and dives down to find a pottery fragment. After being blasted to the surface by a Wailord, Ash and Goh hear a voice calling to them. The voice turns out to come from Drake of the Hoenn Elite Four, who is standing on a salvage tug near them. He and Ash recognize each other, although Ash initially misremembers Drake's name as "Flake" and Drake misremembers Ash's as "Slash".
Climbing onto Drake's boat, Ash and Goh explain to him what they're doing and ask him if he's ever seen a wild Kingdra around here. He says he has, describing how he was once sucked into a whirlpool while diving, leading to the one and only encounter with a wild Kingdra he's ever had. Realizing that Kingdra live far deeper than they expected, Goh asks Drake for help. Drake agrees to lend them his equipment on the condition that they help him in his treasure hunting. Agreeing to this, Ash and Goh stay on the ship while Drake dives down to search for the wreck of an ancient submersible. Goh figures they could help too, and has Ash combine his Lucario's Aura powers with his Chinchou's Supersonic to form a living radar. The search goes on until sunset, when Lucario finally locates something, and Ash and Goh direct Drake towards the setting sun. Arriving at the edge of an underwater ravine, Drake spots the submersible on the other side and takes a few photos of it before asking himself to be retrieved back to the surface, as exploring in the dark could be dangerous.
Back on the ship, Drake offers Ash and Goh a meal and some hot Moomoo Milk while showing them the photos he took of the submersible. It is shaped like a Kingdra, showing that the ancient civilization that built it idolized Kingdra. As further evidence, he shows them an urn with Kingdra-themed decorations that he once found on a beach. Studying it had led him to the legend of an ancient submersible that had sunk with a treasure onboard. This had inspired him to start his treasure hunting hobby whenever he's not busy with his Elite Four duties. Seeing the urn makes Goh realize that the fragment he found earlier was from a similar urn, too. Ash asks Drake what he's going to do after finding the treasure. To the boys' surprise, he says he'll simply start searching for another one, as he's really looking for dreams and adventures.
The next morning, Drake sends Ash and Goh diving down in his stead so that Goh can fulfill his goal of catching a wild Kingdra. He also gives each of them an oxygen tank that contains air for ten minutes should their hoses detach. While the boys approach the submersible, they're momentarily caught in a strong current, causing Drake to realize that the current must've carried the submersible here to begin with, as it hasn't always been on this spot. Entering the submersible, Goh spots a Seadra, but it gets alarmed and swims away before he can catch it. They then find a treasure chest, which, aside from loads of mud, turns out to contain a Dragon Scale, the Evolution item for Seadra. Just then, the Seadra from before returns. Goh pulls out a Poké Ball to catch it, before noticing the Sharpedo that's chasing after it. Surprisingly, Sharpedo ends up hitting Goh's Poké Ball and being caught, saving Seadra.
Noticing Seadra being interested in the Dragon Scale, Goh gives the item to it as a present, before asking it to lead them to Kingdra. Seadra agrees and starts leading the boys towards a cave entrance in the ravine, but their air hoses run out of reach before they can reach it. Suddenly, a whirlpool emerges from the cave and starts pulling on them. Realizing the whirlpool could be Kingdra's doing, Goh detaches his hose and lets the whirlpool suck him into the cave. He attaches his oxygen tank, but as Kingdra is nowhere in sight, he understands that he doesn't have time to wait for another whirlpool to appear. Remembering Kingdra's Pokédex entry, he starts playing a recording of Ash's Dragonite on his Rotom Phone, successfully luring Kingdra out to him. He sends his Inteleon out to battle it, but Kingdra easily dodges all of its Snipe Shots before counterattacking with Dragon Breath. Ash and Dracovish show up to support Goh and keep him from being blown away. Goh calls out his Mantyke and has it use Ice Beam to block off Kingdra's exit. With Kingdra's mobility limited, Goh has Inteleon attack it with Liquidation, before tossing a Poké Ball and having Inteleon launch it with a Snipe Shot, successfully catching Kingdra.
Ash and Goh start heading to the surface, but are caught in the current again just as their oxygen supplies start to run out. Goh calls out Kingdra and has it use Dragon Breath downwards while he and Ash hold on to it, propelling them towards the surface. While speeding towards the surface, Goh notices the current starting to carry the submersible wreck away, with Seadra happily smiling at him from inside while holding its Dragon Scale.
Safely back on the surface, Goh calls Danika, who verifies his capture of a wild Kingdra. She ends the call as she and Quillon approach an ancient ruin in a desert, which opens to reveal a Regirock. While the two Chasers prepare to confront the Legendary Pokémon, Goh receives his second Project Mew token on his Rotom Phone. Heading back to the shore, Drake wishes both Goh and Ash luck in their respective goals and promises that he'll keep hunting for treasures for the rest of his life.