A Celebi flies through a time ripple and stumbles across Ash and his Pokémon in the middle of a practice battle. Pikachu unleashes a powerful Thunderbolt, though Rowlet flies around and evades the attack. It replies with a Seed Bomb, however its Everstone strikes a tree before ricocheting back into Rowlet's head. Rowlet hits Ash's face in its dizzied state. Rotom notices that everyone appears excited and Ash replies that the Pokémon League is just around the corner.
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, however Burnet finds it incredibly adorable. He queries where she found it and Burnet pulls out an old book she found in a storage closet. Kukui is thrilled to see his old "Super Pokémon Move Index" guidebook and reminisces about the times he used to spend watching Hala's battles. As a child Kukui challenged Hala, however the Island Kahuna admitted that he was still too young at the time. Kukui was unfazed and proudly showed off his Firium Z. Burnet's attention soon turns to the Litten also featured in the photo, and Kukui explains that Litten was actually his parent's Pokémon.
Meanwhile Ash's training session wages on. Torracat uses a Fire Blast attack and Lycanroc counters with Stone Edge. The resulting explosion whips up a powerful gust which triggers Celebi to time travel to escape, however it also accidentally transports Ash and Torracat into the past along with it. Ash calls out to Lycanroc and Pikachu, only to get no response. Torracat begins hissing at something in the nearby bushes, and a Litten soon reveals itself. Oddly enough, Torracat appears to recognize the Litten as Kukui's Incineroar, and soon enough, a young boy, the future Professor Kukui, emerges from the bushes. Kukui is thrilled to see a Torracat in-person and tries to pat it, however Torracat steps back and begins raising the hairs on its back in anger. As Ash pats Torracat to settle it, Kukui spots Ash's Z-Power Ring and asks if he is taking part 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. Ash has his doubts, but Torracat reassures him otherwise. He borrows Kukui's Firium Z and prepares to aim an attack at a rock, which he seems to not remember. Ash takes a moment to recall how Kiawe performs the Z-Move and, to his own surprise, his own attempt at it works. The Inferno Overdrive destroys the target rock into small fragments. Kukui is thrilled by powerful display and gives Ash the nickname "Move King". He then challenges Ash to a battle, and he happily accepts.
A crowd of wild Pokémon have assembled. Ash whispers to Torracat to go easy on the young boy's Litten, however Kukui demands not to be treated like a kid. Ash adheres to the young boy's request and agrees to use full power moves only. He nicknames the boy "Gao" because of his signature Incineroar hat. Kukui takes the nickname in his stride as he issues his first attack, an Ember. Torracat simply swats away the move with its front paw before with a Flame Charge. Ash orders a follow-up Fire Blast. Gao struggles to think of a counter attack and soon Litten is knocked out. He tears at his loss, but Litten begins licking his face to cheer him up. Ash walks over and congratulates Litten on its efforts, and the little Fire-type smiles back. He tells Gao that they can battle another time, but Gao declares that using fire Blast was unfair. Ash admits that it was Gao who wanted a full power battle. The young boy is upset by his loss, but Ash declares that there is always the chance he can win next time. The advice instantly perks Gao up, and he tells Litten that they will continue to get stronger and one day beat the Move King.
Torracat tugs on Ash's shorts to inform him that they are being watched. Gao admits the wild Pokémon are his friends and waves them over. The Pokémon quickly rush out of the woods to swarm the two Trainers. Shortly after a Totem Trevenant approaches them. Gao rushes towards Trevenant, referring to it as "Grandpa Oh". As he leaps onto one of Grandpa Oh's roots, Gao admits that the Totem Pokémon gifted him the Firium Z. Grandpa suddenly sneezes, so Gao has Litten use Scratch to ease its symptoms. Torracat is keen to assist, but Gao explains that Litten's claws are small enough to help with Trevenant's itch.
Suddenly dark clouds begin to build followed by a rainstorm. To combat the wet conditions, Trevenant manipulates the trees with a special power into shelter for all the Pokémon, Ash and Gao. Ash's attention turns to the book in Gao's hands, and he explains that it is his trusty move index. Gao declares that seeing moves in-person is much better and queries Ash on how Torracat learned Fire Blast. Ash admits that it is simply practice. Gao is curious about Ash's battle experiences, and Ash explains that before coming to Alola he travelled to many cities to compete in Gyms and battle their resident Gym Leaders. 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 instantly thrills Kukui, and he queries what happens after you win a League. Ash doesn't answer, instead he adds that he wants to become a Pokémon Master. Gao doesn’t understand the concept but declares that he wants to become a Master.
The rain suddenly eases and the sunlight illuminates the area once again. Ash and Kukui thank Grandpa Oh for its help. However, Trevenant begins sneezing again. Litten rushes to Scratch its back, but the cure doesn't work this time. Litten urges Torracat to assist, and together the pair climb up into Trevenant's head canopy. Ash quickly reaches the top of Trevenant's head, and with a helpful lift from a Heracross, Gao also joins him. Ash and Gao soon spot the problem, a horde of Cutiefly seeking refuge. He and Ash lift them out of Trevenant's shrubbery before using their Fire Pokémon to dry off their wet wings. The Cutiefly soon flutter away and Trevenant is feeling relieved. Grandpa Oh produces a Firium Z, and it lands in Ash's hand. Gao admits that he got his Z-Crystal after he first helped Grandpa Oh, and so Ash accepts the gift.
Celebi soon flies by and notices an aura radiating from Ash and Torracat. As Ash and Torracat leap down from Trevenant's head, Celebi whisks the pair back to their own time. Gao lands on the ground, only to notice Ash’s mysterious absence, though Trevenant and Litten are unable to offer any insight. Back in his own time, Ash turns to Grandpa Oh, but notices that only a tree trunk with overgrown remains. Ash's other Pokémon rush over to greet their Trainer, greatly relieved to see him back. Ash notices that Lycanroc and the others appear particularly dirty, but he is unable to understand their Pokémon speech replies. Rotom then returns after trying to chase down Celebi for a snapshot.
Later, Ash and his Pokémon return home to see Burnet preparing lunch. She spots his dirty hands and orders Ash to wash his hands at once before calling Kukui in for the meal. As Ash follows through, Rotom happily shows Burnet its picture of Celebi. She is startled to see the Mythical Pokémon, admitting that 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 up his move guidebook.