Ash and his classmates are out fishing for the day. As Kiawe becomes frustrated over his inability to catch anything, Mallow reels in a Bruxish, but it decides to let go of the line and tries to bite Mallow, forcing Tsareena to kick it away. Meanwhile, Sophocles bides his time and watches his digital screen before pulling up his catch, a particularly large Pyukumuku. Sophocles's catch proves to be too heavy, and Pyukumuku latches onto Kiawe's face before it is returned. Then, Turtonator feels something tugging on its tail and pulls it up to reveal an Octillery, which only annoys Kiawe more. Sophocles notices that Lana and Ash are missing, but Lillie points them out in the distance. The pair is floating across the ocean in two of Brionne's bubbles. Ash calls for a race, and Lana accepts. Brionne promptly dives underwater and gives Lana a push forward. Ash chases her, leaving their friends behind.
Later on, Kiawe finally hooks something. However, it is particularly strong and begins to pull Kiawe towards the water. His friends and the Pokémon add their strength, but they continue to struggle. Kiawe finally heaves his catch, a Cloyster, out of the water, but it immediately clamps onto Kiawe's head. Tsareena kicks Cloyster back into the depths, freeing Kiawe from suffocation.
Meanwhile, Lana and Ash are left exhausted from their race and take a moment to catch their breaths. Suddenly, a giant whirlpool begins to swirl, sending the pair into the vortex. Brionne does its ample best to reach Lana, but a Kyogre swims past them. In its wake, Lana and Ash are sent flying onto a rocky exposed reef, soon enough Brionne joins them ashore. Ash turns to see a steep cliff, so he and Pikachu climb it with ease. With their heightened vantage point the pair spot a Kyogre staring back at them. Lana soon joins them and thrilled to see Kyogre. She suddenly notices that it glowing purple, red, and blue as well as breathing heavily, so she assumes it must be unwell. The pair rush down and Pikachu is immediately concerned by Kyogre’s ill state. Lana grabs her fishing rod with one of Misty's special lures attached to the end. Ash is surprised to see Lana having one, and she explains that Misty gave the lure to her recently. Lana attaches some bait to the lure, laced with Antidote to heal Kyogre's suspected poisoning. She throws out her line, but Kyogre doesn't take to it. Lana urges Kyogre to grab the bait when a Seismitoad attacks her.
Ash and Lana look around them as a Carvanha submarine suddenly appears. A hunter named Hunter D emerges from the hull and with a snap of his fingers, a pair of crew members and their Palpitoad appear. The crew members snare Kyogre in an electric net, inflicting significant pain onto the Legendary Pokémon. The leader radios into his grunts before capturing Kyogre in a yellow beam and beginning to drag it further out to sea. Ash informs Lana that a Pokémon Hunter is someone who captures Pokémon for a profit. Lana is immediately irritated and prepares to cast out her line to save Kyogre from the mistreatment. Hunter D's Seismitoad leaps towards her, though Sandy batters it away. Seismitoad attempts to use Venoshock, though Brionne strikes back with a Bubble Beam as Sandy knocks it into a nearby cliff. Lana thanks her Pokémon for their efforts and begins to walk towards Kyogre again. Hunter D is furious and orders his crew members to surround the interfering duo. Ash calls out all of his Pokémon to join the fight. Lycanroc and Torracat rush ahead, evading a Mud Shot in the process. Lycanroc unleashes a Stone Edge, but the Palpitoad back-flips to evade the attack. Torracat, however, knocks it into the water with a Fire Blast. The second crew member orders a Hydro Pump which Pikachu narrowly sidesteps to avoid. Pikachu crashes into Palpitoad with a Quick Attack, and Sandy uses the cliff's heights to land a powerful overhead Double-Edge. Ash turns his attention to Hunter D's submarine and has Rowlet strike it with a Seed Bomb. Rowlet promptly swallows its precious Everstone again as Meltan lands onto the submarine.
Lana throws her line again and pleads for Kyogre to grab a hold of it. Hunter D continues to pull Kyogre out to sea, he then orders his Seismitoad to attack Lana. Pikachu leaps to Lana's aid with an Iron Tail and knocks Seismitoad into a cliff with its own Focus Blast attack. Lana tries again, and this time Kyogre takes a large mouthful of water and chomps onto the lure. She maintains her grip as she is now being dragged out as well. The Palpitoad duo attempt another ambush, but Torracat's Flame Charge and Lycanroc's Accelerock batters the threats away. Ash and Lana's other Pokémon continue to fight off the Palpitoad and Seismitoad, while Lana holds steady despite the increasing wake. Meltan's taste for metal proves useful as it begins to eat its way through Hunter D's metal sub. One of the crew members inside the sub radios into the boss to alert him that the tractor beam is quickly losing power. Hunter D calls for a change of plans; trap Kyogre inside the sub. The Carvanha submarine stops its tractor beam and hinges its jaw wide open to grab the target. However, Kyogre appears to have regained some of its strength. The Legendary lets out a mighty roar which summons dark clouds that cover the sky. A torrential rain begins to fall, and the sudden change of conditions also draws Lillie and the others' attention further away. Ash quickly recalls all of his Pokémon to their Poké Balls. Kyogre leaps from the water and the resulting wake washes over Ash and Lana. Brionne saves Ash, Pikachu, and Sandy with its bubbles. As for Lana, she continues to hold onto her line and Kyogre, who soon drags her into the ocean depths. It gives Hunter D's submarine a flick with its tail and sends them blasting off. Hunter D and his two crew members land directly in front of Officer Jenny, who was enjoying a day on the beach.
Meanwhile, Brionne attempts to reach Lana, but Kyogre's speed is unparalleled. In its desperation, Brionne bursts into brilliant blue light and promptly evolves into a Primarina. Kyogre suddenly lets go of the fishing line and Primarina goes on to rescue Lana. Ash, Pikachu, and Sandy watch in amazement as Primarina emerges from the water in a vertical leap. Kyogre suddenly freezes over the water using Sheer Cold to create an icy refuge from the swirling seas. Lana realizes that Kyogre is still angry and sets herself the task of finally reeling the Legendary in. Ash is left baffled when Lana climbs aboard Primarina and simply leaps back into the treacherous seas. Lana casts her line once more and soon enough Kyogre bites onto it. She urges Primarina to keep swimming as she tries to hold onto her big catch. Kyogre makes the task challenging as it performs bounding leaps and tossing rolls. Primarina maneuvers Lana without any trouble. Lana suddenly reels Kyogre into the air and orders Primarina to sing. Primarina performs a melodic opera that calms Kyogre as it leaps over the duo. The subsequent wake ends the performance and the skies instantly clear. Later, Lana and Primarina swim out to their new friend Kyogre and they thank it for the precious moment. Kyogre swims back and with a roar it destroys its Sheer Cold island, sending Ash, Sandy, and Pikachu into the water. A spectacular sprinkle of ice concludes the encounter.
Lana and Ash return to their friends, however, they believe Lana's latest story is just another one of her jokes. She assures them of her miraculous tale and proudly presents her newly-evolved Primarina. Ash goes into detail about the encounter with some onomatopoeia, instantly hooking his classmates into the elaborate story. Rotom is disappointed that it missed out on recording everything.