Ash and Goh have arrived at Stow-on-Side, their visit inspired by Ash finding out that Gengar can Gigantamax. Eager to Gigantamax his own Gengar, Ash is following Professor Cerise's advice to meet Stow-on-Side's Ghost-type Gym Leader, Allister. However, when Ash and Goh arrive at Allister's Ghost-type Gym, they discover it's an overgrown building that pales in comparison to Bea's Stadium. Seeing no entrance, Ash loudly calls out that he's come to ask something regarding his Gengar. Allister opens the door, and Ash and Goh are surprised to find out he's the boy they had met aboard the Ghost Train.
Inside the Gym, Ash explains to Allister that he wants to Gigantamax his Gengar. Since the Ghost-type Gym is too cramped, Allister leads them to the Stow-on-Side Stadium. On their way in, they encounter Bea. She asks if Ash has come to battle, but Ash explains his intention on doing Gigantamax training instead. Bea is surprised to see Allister helping someone, but nevertheless wholeheartedly allows them to use the Stadium for the training. At the battlefield, Ash recalls Gengar and uses his Dynamax Band on its Poké Ball before sending it back out. However, instead of Gigantamaxing, Gengar ends up only performing a regular Dynamax. Ash tries to have Gengar transform into its Gigantamax form, but Allister tells him that it can't Gigantamax, no matter how hard it's pushed. Dismayed, Gengar returns to its normal size. Allister says there is a solution, though: if they collect Max Mushrooms and cook them to make Max Soup, Gengar could become able to Gigantamax. Ash is excited, and, not too afraid of Allister's warning about the forest where the mushrooms grow being populated by Ghost Pokémon, pulls him along as he heads out, followed by Gengar and Goh.
The first cluster of Max Mushrooms they find grows in an old graveyard. Allister warns them that some of the stones that litter the graveyard are actually Galarian Corsola, which will curse anyone who disturbs them. Ash figures out a way to get to the mushrooms without disturbing any of the Corsola by clinging to Gengar as it carries him across the graveyard. He manages to pick up the mushrooms, but drops his hat in the process. It hits a rock, which awakens and reveals itself to be a Galarian Corsola. Goh starts to panic and quickly catches Corsola to save Ash from being cursed. Ash is happy for them getting the mushrooms, but Allister says they don't have enough to make Max Soup yet. As such, they wander deeper into the forest, where they find another cluster of mushrooms and only barely manage to pick it up before they're attacked by a Trevenant that hates humans disturbing its forest.
After escaping from Trevenant, Allister tells them they still need a little more Max Mushrooms. Fortunately, Gengar quickly spots another cluster nearby, except that a sleeping Drakloak is using it as a pillow. When they examine it closer, they see the Dreepy on its head slipping off. When Ash catches Dreepy, the Lingering Pokémon finds a new sleeping place under his hat. When Drakloak notices Dreepy is missing, it picks up Grookey to use as a replacement. Grookey, as its usual habit, starts hitting Drakloak on the head with its stick. The Caretaker Pokémon is aggravated by this and starts wildly flying around in the air. Allister says they need to return Dreepy, but when Ash tries to do so, Drakloak angrily dives towards him, and he's only narrowly saved by Gengar catching Drakloak. Grookey is recovered, Dreepy is returned to its proper place, and Drakloak agrees to give the mushroom to Ash, finally giving them enough of them to make Max Soup.
Back at the Ghost-type Gym, Allister puts the Max Mushrooms into a pot and says they must now let the soup stew overnight. Allister has a talk with Gengar, reminding Ash and Goh of his ability to understand Ghost-type Pokémon. Allister is left flustered when the boys, as opposed to being weirded out by his ability, actually find it interesting. Ash promises to stay watching over the pot, but ends up falling asleep as the night comes. Allister lets Goh catch some sleep too while he watches over the pot. When Gengar tries to open the pot out of curiosity, Allister tells it not to do so. He then talks with Gengar about how he now understands why it wants to become stronger. It's been through lonely times, but now that it's with Ash, a Trainer who cares about and wants to work hard together with it, things are looking better. Gengar is happy at Allister understanding it and hugs him. Ash and Goh, who are half awake, smile at this.
The next day, the Max Soup is ready, and Gengar eagerly swallows the whole potful of it. Allister then leads them to the Stadium again, where Bea shows up again to watch as Ash once more recalls Gengar and uses his Dynamax Band on its Poké Ball. This time, though, Gengar successfully Gigantamaxes, much to Ash and Goh's joy. Allister is also delighted to see Gengar having achieved its goal of becoming stronger. Later, when Ash and Goh board a train departing from Stow-on-Side, Allister comes to see them off. Ash thanks Allister for his helps and expresses his hopes for getting to meet him again in the future. As the train departs and leaves, Allister waves after it, hoping for the same.