A Celebi flies through a time ripple and stumbles across Ash and his Pokémon while they are in the middle of a practice battle. Meanwhile, over at the Professors' residence, Professor Burnet shows her husband one of his childhood photos. Kukui is embarrassed by it, admitting that he was only five years old at the time, but Burnet finds it adorable. He asks where she found it and Burnet pulls out an old book she found in a storage closet. Kukui is thrilled to see his old "Book of Pokémon Super Moves" and reminisces about the times he used to spend watching Hala's battles. He reveals that, as a child, he tried to challenge Hala, but the Island Kahuna admitted that he was still too young at the time. Kukui, however, was unfazed and proudly showed off his Firium Z. Burnet then spots the Litten in the photo, and Kukui explains that Litten was actually his parent's Pokémon.
Meanwhile, Torracat uses Fire Blast and Lycanroc counters with Stone Edge. The resulting explosion whips up a powerful gust that prompts Celebi to time travel in order to escape. However, it also accidentally transports Ash and Torracat into the past along with it. Torracat encounters a Litten and recognizes it as Kukui's Incineroar, after which a younger Kukui emerges from the bushes. Kukui is thrilled to see a Torracat in person and tries to pat it, but Torracat is intimidated and distances itself. As Ash pats Torracat to settle it down, Kukui spots Ash's Z-Power Ring and asks if he is participating in the island challenge. Kukui shows off his own Firium Z and Ash admits that he doesn't even have that Z-Crystal yet. The young Kukui declares himself the winner before asking Ash to show him a fully-fledged Inferno Overdrive. Though initially hesitant, Ash borrows Kukui's Firium Z and aims it an attack at a rock. Ash takes a moment to recall how Kiawe performs the Z-Move and, to his surprise, his own attempt at it works. The Inferno Overdrive destroys the rock into small fragments, astounding Kukui, who gives Ash the nickname "Move Kahuna". He then challenges Ash to a battle, and Ash accepts.
As a crowd of wild Pokémon assembles around them, Ash nicknames Kukui "Roarie" because of his signature Incineroar hat, which Kukui accepts. He orders Litten to use Ember, but Torracat swats away the move with its front paw before using Flame Charge and following up with Fire Blast. Roarie struggles to think of a counterattack, but Litten is quickly knocked out. He starts crying at his loss, but Litten begins licking his face to cheer him up. Ash walks over and tells Roarie that they can battle another time, but Roarie claims using Fire Blast was unfair. Ash admits that it was Roarie who wanted a full-power battle but says there is always the chance of Roarie winning next time. The advice invigorates Roarie, who tells Litten that they will continue to get strong enough to beat the Move King.
Torracat tugs on Ash's shorts to alert him that they are being watched, but Roarie admits the wild Pokémon are his friends and waves them over. The Pokémon quickly run out of the woods and surround the two Trainers. Shortly after, a Totem Trevenant approaches them. Roarie runs towards Trevenant, referring to it as Elder. As he leaps onto one of Elder's roots, Roarie admits that the Totem Pokémon gifted him with the Firium Z. Elder suddenly sneezes, and Roarie has Litten use Scratch to ease its symptoms.
Suddenly, a rainstorm approaches, forcing Elder to manipulate the trees into forming a shelter for all of the Pokémon, Ash, and Roarie. Ash's attention turns to the book in Roarie's hands, and he explains it is his move index. Roarie says seeing moves in-person is much better and asks Ash how Torracat learned Fire Blast. When Ash says he practiced, Roarie is curious about Ash's battling experiences, and Ash explains that before coming to Alola, he traveled to many cities to compete in Gyms. Ash adds that a successful Trainer earns a Badge, and with enough Badges, they can compete in the region's Pokémon League. The sound of a battle festival excites Roarie, and he asks what happens after if one wins a League. Ash doesn't answer, instead, he mentions his goal of becoming a Pokémon Master. Roarie doesn't understand the concept but declares that he wants to become a Master too.
When the rain recedes, Ash and Roarie thank Elder for its help, but it begins sneezing again and Scratch doesn't work this time. Litten urges Torracat to assist it, and together, the pair climb up into Elder's head canopy. Ash and Roarie also reach the top of Elder's head, where they find a horde of Cutiefly seeking refuge inside. He and Ash lift them out of Elder's shrubbery before using their Fire Pokémon to dry off their wet wings. The Cutiefly fly away and Elder is feeling relieved. Elder produces a Firium Z, and it lands in Ash's hand. Roarie admits that he got his Z-Crystal after he first helped Elder, and as a result, Ash accepts the gift.
Celebi soon flies by and notices an aura radiating from Ash and Torracat. It promptly whisks the pair back to their own timeline, which is not witnessed by Roarie, who is perplexed by the occurrence. Back in his own timeline, Ash finds a tree trunk with overgrown remains in Elder's place. Ash's other Pokémon greet their Trainer, and Ash notices that Lycanroc and the others are dirty. Rotom then returns after trying to chase down Celebi for a snapshot. The group then returns home to see Burnet preparing lunch. Rotom happily shows Burnet its picture of Celebi; she is startled to see the Mythical Pokémon, believing it must have been drawn to the forest's natural beauty. Meanwhile, Ash calls Kukui over for lunch. Kukui slots in his childhood photo as a bookmark before closing his move guidebook.