When Lillie’s big brother Gladion arrives on Melemele Island, everyone is excited about the powerful new Trainer in town—particularly Ash, who really wants to battle him! After hearing about Ash’s Z-Ring and his battles against Tapu Koko, Gladion agrees to meet up the next morning.
The battle is just getting started, with Ash’s Rockruff against Gladion’s impressive Lycanroc, when Team Rocket swoops in. Gladion and Lycanroc unleash a new Z-Move to send them blasting off again, and Ash is immediately interested in continuing the island challenge so he can get a Rock-type Z-Crystal!
School is starting, so the battle is postponed—but it seems Ash has a new rival!
Team Skull Grunts Tupp, Rapp, and Zipp are in an alleyway when Gladion walks past along with his Umbreon. They confront Gladion, but Gladion keeps on walking and demand that he leave his Umbreon with them. They send out their Pokémon to battle as Gladion sends out his Lycanroc. Team Skull orders their Pokémon to attack, but Lycanroc knocks them all out with one move. They recall their Pokémon and retreat as Gladion recalls Lycanroc. Team Rocket witnessed his battle and realizing how powerful his Lycanroc is decide to steal it. Outside Professor Kukui's house, Ash is training with his Pokémon with Rockruff battling against Rowlet. After a battle, Ash tells Rockruff to take a break and has Litten come into battle, only for Rockruff to jump over Litten to hit Rowlet. Seeing that Rockruff is fired up to battle, Ash has Rowlet take a break. Soon, Ash decides to end the battle, and Rockruff rubs Ash affectionately as they head to school.
During their lunch break, Ash and his classmates watch their Pokémon play in the classroom having formed a train. They discuss reports which include the trainer having a Midnight Form Lycanroc and an Umbreon. Ash says he would love to battle that trainer, and hopes it will happen someday. In town, Gladion is battling against a Sailor and his Blastoise, where despite a type disadvantage, Lycanroc is holding its own. Nearby, Ash is doing some shopping before noticing the battle and they go down for a closer look. They arrive and watch as Lycanroc knocks Blastoise out. Rotom tells Ash that as Gladion has Lycanroc and Umbreon, he must be the mysterious trainer. Ash runs over to Gladion as he recalls Lycanroc and introduces himself as someone calls Gladion's name. They both look and see Lillie arrive with Snowy and Hobbes. Lillie runs over to Gladion, revealing him to be her older brother.
Gladion sees that Lillie can touch Pokémon, which he is glad to see. Lillie introduces him to Snowy, and Hobbes tells Gladion that she has been raising Snowy since it was an egg. Gladion tells Lillie to take care of Snowy and goes to leave but Hobbes asks him to come with them to the mansion. Gladion declines as Ash tries to get Gladion to have a battle. Gladion notices Ash's Z-Ring and learns that he got it from Tapu Koko. Gladion tells Ash that he'll consider having a battle and leaves as Ash tells him that he is staying at Professor Kukui's house. Later at Lillie's mansion, Lillie tells Ash that Gladion left home having wanted to train by himself and Hobbes says that Gladion wanted to improve on his battling skills. Lillie notes that Gladion has changed. Hobbes remembers when Gladion had brought an injured Eevee home, he helped take it to Nurse Joy.
That night, Ash has told Professor Kukui about his encounter with Gladion. Rockruff is becoming more active and Professor Kukui guesses that Rockruff has become a fan of the Lycanroc. Later, everyone has fallen asleep but Pikachu is awoken by knocking and looks up to see Umbreon on top of the roof. Pikachu wakes Ash up and points to the window where Umbreon drops a note which Ash reads and becomes excited as Rotom asks what it is about. Jessie leads her teammates along the beach and notice Gladion and Umbreon and recognize him from before. They watch as Ash arrives and meets up with Gladion who informs him that he asked him here to test his battling skills. Team Rocket looks on as Gladion has a battle with Ash who chooses Rockruff as Gladion sends out Lycanroc. Jessie gets an idea to use this opportunity to catch Lycanroc and Pikachu. The battle begins with Rockruff using Bite. Rockruff hits Lycanroc before being thrown away. Rock Throw is countered by Lycanroc's Stone Edge which hits Rockruff.
As Rockruff gets up, ready to battle, Team Rocket fire two nets that capture Lycanroc and Pikachu. As Gladion asks who they are, they recite their motto as Ash demands they give them back their Pokémon. Team Rocket drives off in a truck as Ash, Gladion, and Rockruff chase after them. Gladion has Umbreon use Dark Pulse which flattens a tire, sending the truck out of control as Rockruff frees Pikachu and Lycanroc. Jessie isn't happy at them getting away and is confronted by Pikachu and Lycanroc. Gladion decides to deal with Team Rocket and they power up to use Continental Crush. Lycanroc hits them with Continental Crush which sends them blasting off only for Bewear to push itself into the air using the trees and catches them and takes them back to its den. Amazed, Ash tells Gladion that he wants to try a Rock-type Z-Move.
After learning that Ash doesn't have the Rock-type Z-Crystal, Gladion tells him to go to Akala Island and pass its grand trial. As they go to finish their battle, Lillie arrives and Rotom reminds Ash that it is nearly time for School as Ash realizes how late it is as Gladion tells them they'll have to finish it later and he finds it difficult with Lillie watching. Ash agrees. Gladion leaves as Lillie comes over to Ash, asking where Gladion is, only to learn that he has left and she was hoping to force him to come home for one day.