In a locker room at Wyndon Stadium, Marnie is looking forward to her one-on-one World Coronation Series battle against Ash. Ash, however, has instead just arrived at Spikemuth with Goh, a group of people calling themselves Team Yell having told them that the battle would be held there. They enter the local Gym, where they find Piers rehearsing with the Maximizers band. He is dissatisfied with the interruption, but nevertheless proceeds to introduce himself as the local Dark-type Gym Leader, and the Pokémon as his fellow band members. When Ash introduces himself, Piers recognizes him, since Ash's next World Coronation Series opponent, Marnie, is actually his sister. He's also confused of why Ash is here instead of Wyndon Stadium, where Marnie is waiting for him. This revelation causes Ash to realize that Team Yell had lied to him, and grows all the more alarmed when Goh tells him that they don't have much time to get to Wyndon.
Fortunately for Ash, Piers is ready to take full responsibility for Team Yell's actions and agrees to take Ash to Wyndon on his motorcycle. However, he also proceeds to drive quite slowly for safety reasons. As they approach Wyndon, Piers reveals to Ash that Team Yell is made up of Spikemuth Gym Trainers who are fans of Marnie and ready to support her in any way they can, like how they had lied to Ash regarding the location of his and Marnie's battle. While Ash says he'd be happy to battle at the Spikemuth Gym, Piers explains that Marnie can't use her full potential there, since the Spikemuth Gym lacks a Power Spot required for Dynamaxing. This suits Piers just fine, since he prefers regular battles and doesn't use Dynamax, but it has also made the Spikemuth Gym unpopular and led to the town becoming almost completely deserted. Marnie had entered the World Coronation Series in the hopes of winning it to make Spikemuth more famous and revitalize it. Team Yell is hoping for this as well, but Piers blames himself for not taking on the duty himself.
At Wyndon Stadium, the scheduled battle time has come and gone, but Marnie refuses the idea of winning by default. Fortunately, Ash enters the battlefield just in time to avoid being disqualified. Team Yell, being part of the audience, berates Ash for making Marnie wait, but he snaps back at them for being the reason behind his lateness in the first place. Learning this, Marnie berates her fans, since winning through default wouldn't be a real victory in her eyes. Piers also appears next to Team Yell and makes his presence known by shouting out encouragements to his sister. Both Ash and Marnie express their resolve to win, with Ash taking his place at his own end of the battlefield just as Goh joins Piers in the audience, having made it here just in time thanks to a Flying Taxi.
At the Drone Rotom's signal, Ash and Marnie send out their respective Pokémon, Gengar and Grimmsnarl, the latter of which Piers acknowledges as Marnie's ace. Marnie makes the first move by ordering a Spirit Break, which powers through Gengar's Dazzling Gleam, but still ends up missing and being countered by an Ice Punch, which sends Grimmsnarl crashing to the ground. When Gengar prepares to use Shadow Ball, Grimmsnarl suddenly bows before it, catching Gengar off-guard and leaving it open for Grimmsnarl's signature move, False Surrender, dealing super effective damage on Gengar. Marnie then takes the battle to the next phase by recalling Grimmsnarl and using her Dynamax Band on his Dusk Ball, calling forth Gigantamax Grimmsnarl. In the audience, Piers explains to Goh that Dynamax battles, the one thing that cannot be done at the Spikemuth Gym, are Marnie's specialty, showing that both she and Piers have their own ways of helping their hometown. Marnie calls out for Grimmsnarl's G-Max Move, G-Max Snooze, which Gengar, through much effort, manages to block with Dazzling Gleam. G-Max Snooze's secondary effect causes Gengar to grow drowsy, but it manages to resist falling asleep, to Marnie's surprise.
Gengar starts phasing in and out of shadows around Grimmsnarl, which Marnie responds to by telling her Gigantamaxed partner to use Max Quake. The Max Move strikes Gengar, whose attempted Sludge Bomb counterattack ends up doing next to nothing to Gigantamax Grimmsnarl. Ash decides to even the odds and Gigantamaxes Gengar as well. Following a clash between Max Ooze and Max Starfall, Grimmsnarl's time in his Gigantamax form runs out and he reverts to his normal size. Despite this, he still successfully avoids a Max Hailstorm and strikes Gengar with Spirit Break, before charging in with Darkest Lariat. Gengar attacks with G-Max Terror, but the Darkest Lariat protects Grimmsnarl as he keeps charging in. Not wanting to leave Gengar vulnerable as its Gigantamax wears off, Ash has it swallow Grimmsnarl, trapping the Bulk Up Pokémon in Gengar's mouth as it reverts to normal size and spits him back out. Marnie orders Grimmsnarl to use Stomping Tantrum, a move that grows in power if the user's previous move failed to hit. Ash has Gengar counterattack with Shadow Ball, which powers through Stomping Tantrum and strikes Grimmsnarl, taking him out of the fight and making Ash the winner. Both Ash and Marnie compliment their respective partners for a good fight, which the audience also joins in, congratulating Marnie's performance as well, despite her loss. With this victory, Ash climbs to rank 15 in the World Coronation Series.
Later, outside the stadium, as both Ash and Marnie compliment each other, Team Yell approaches Marnie and excitedly tells her that many people were so impressed by her performance that they're now wanting to visit Spikemuth, providing hope in revitalizing their hometown. Marnie pulls up a wide smile to celebrate the occasion and shakes hands with Ash, promising to do her best in the future as well. Ash happily agrees, his victory having brought him closer to the Master Class than ever before.