At Professor Kukui's house, Ash is serving Pikachu and Rockruff some Pokémon food, as Kukui, reading a newspaper, notices an article about Ash's achievement the previous day. Rotom starts reading the article out loud, informing everyone in the room that Ash is due to receive a certificate from Officer Jenny as a recognition of his work. Just then the doorbell rings, and Ash rushes to open the door. Opening the door, he comes face to face with Officer Jenny and Gumshoos. She thanks Ash for helping her find a partner for herself, and Rotom states that Gumshoos is a perfect Pokémon to be Jenny's partner. As Ash is due to have his grand trial against Hala later that day, Jenny offers to drive Ash and Kukui to Iki Town, which they gladly accept. Kukui then reminds that Jenny also had another reason to come, and she, reminded of the fact, hands Ash the certificate promised to him.
Later in Iki Town, Officer Jenny departs to continue her duties, while Ash and Hala are praying for Tapu Koko's blessing for their upcoming battle at the Ruins of Conflict. Ash is getting uncomfortable for having to keep sitting still, but eventually Hala tells that they're done here. Ash asks if Tapu Koko heard them, and Hala answers that it's not certain, as Tapu Koko is a very fickle creature. Just then, the two hear Tapu Koko's cry echo from afar, confirming that it had been listening to them.
Back in Iki Town, Professor Kukui asks Ash if he remembers how to do the Normal-type Z-Move pose, and Ash assures him that he has practiced it well enough to remember it. Kukui also warns Ash that a Z-Move can only be used once per battle, meaning that the moment to use it must be chosen carefully. With technicalities out of the way, Kukui takes his position as the referee of the grand trial battle. As the battle is about to begin, Ash sends out Rowlet, which Rotom is surprised to see being wide awake when coming out of its Poké Ball. Hala sends out his own Pokémon, a Crabrawler, which Rotom gives Ash information about, not forgetting to mention that since Rowlet is a Flying Pokémon, it has a type advantage over the Fighting-type Crabrawler. Crabrawler starts the battle with a Bubble Beam, which Rowlet avoids before countering with a Peck attack, dealing super effective damage on its opponent. Ash orders Rowlet to use Peck again, but Crabrawler manages to grab Rowlet before the attack can connect. By Hala's command, the Boxing Pokémon then uses Brutal Swing, sending Rowlet flying into the air, where it then gets struck by a powerful Power-Up Punch. Rotom is worried about Rowlet having taken a lot of damage, while Kukui comments how Hala's Crabrawler is just as strong as ever.
Up in the air, Rowlet manages to get itself back together, and Ash tells it to use Leafage, which, much to Ash's shock, gets completely blocked by Crabrawler's Bubble Beam. While Ash marvels his opponent's strength, Rowlet suddenly sneaks up behind him, causing Ash to jump. This, however, gives him an idea, and he tells Rowlet to use Leafage and sneak up behind Crabrawler. The tactic succeeds, allowing Rowlet to land a surprising Peck attack on Crabrawler. A flurry of more Pecks follows, Crabrawler being unable to dodge any of them. Finally Hala calls Crabrawler to block Rowlet's attack with its pincer, allowing it to land a Power-Up Punch with the other pincer. Hala follows up with a command for Brutal Swing, but Rowlet manages to avoid the attack and strike back with Tackle, sending Crabrawler slamming into the ground and causing it to faint. As Hala recalls Crabrawler, Ash and his partners congratulate Rowlet on its victory. Suddenly Rowlet starts snoring, and Rotom, upon closer inspection, informs Ash that Rowlet has fallen asleep and is now sleep-flying. While Ash, Pikachu, and Rotom attempt to wake Rowlet up, Hala calls his Hariyama to the battlefield. All attempts to wake up the tuckered-out Rowlet fail, leaving Ash with no choice but to recall it. Rotom provides Ash with basic information about Hariyama, mentioning that it's able to send a 10-ton truck flying with a single punch, which is equal to the weight of 1667 Pikachu.
Both Pikachu and Hariyama prepare for battle. Ash takes the initiative by having Pikachu use Iron Tail, but Hariyama counters with Fake Out, grabbing Pikachu by the tail and stopping the attack. Hala continues by having Hariyama use Knock Off, which sends Pikachu straight into the ground. As Kukui compliments on Hala's combination attack, Ash has Pikachu use Electro Ball, but the attack is stopped by Hariyama's Arm Thrust, which then proceeds to repeatedly strike Pikachu, dealing a lot of damage on the Mouse Pokémon. Finally Ash finds an opening, and has Pikachu slip past Hariyama before counterattacking with Thunderbolt. However, even though the attack makes a clean hit, it doesn't seem to hurt Hariyama too much. Ash is getting nervous, thinking about using his Z-Move, but he also acknowledges the fact that he only has one shot at using it. Hala then commands Hariyama to use Belly Drum, which Kukui considers to be a ruthless move on Hala's part, as it increases the user's Attack power to its maximum, but also takes away half of their health. With the power-up complete, Hala prepares to use his own Z-Move, striking up synchronized poses with Hariyama as energy flows from his Z-Crystal to his Pokémon. Ash tells Pikachu to use Quick Attack to avoid the upcoming attack, just as Hala and Hariyama unleash the Fighting-type Z-Move: All-Out Pummeling. Pikachu does his best to avoid the Z-Move, but is ultimately hit, sending him flying backwards. Despite this powerful hit, however, Pikachu is still capable of getting back to his feet, impressing Hala, who says that he should've expected as much from someone whom Tapu Koko has taken interest in.
Ash, following his usual attitude, isn't giving up, and has Pikachu charge at Hariyama with Quick Attack and run around the Arm Thrust Pokémon. While Hariyama is unable to get a clear fix on Pikachu, Ash has Pikachu use Iron Tail, striking Hariyama on the leg and causing it to fall on its knee. Seizing the chance, Ash and Pikachu prepare to use their own Z-Move, striking up their well-practiced and well-synchronized poses, causing Ash's Normalium Z to glow and charge Pikachu up with Z-Power. Feeling the power that connects him and Pikachu, Ash has Pikachu unleash its Normal-type Z-Move: Breakneck Blitz. The powerful attack hits Hariyama head-on, sending the huge Pokémon flying out of the field. While Ash and Pikachu celebrate successfully using their Z-Move, Kukui sees that Hariyama has fainted, making Ash the winner of the battle. After recalling his defeated Pokémon, Hala turns to Ash, congratulating him and his Pokémon on the well-fought battle and the joyful way he used the Z-Move. As the Kahuna of Melemele Island, Hala declares Ash having passed his grand trial, which Kukui decides to hold a celebration for. Hala reaches inside his sleeve and presents Ash with a Fightinium Z as a reward for defeating him. Ash thanks Hala for his gift, but just as he reaches out to take the Z-Crystal, a cry echoes across the landscape, and something blindingly fast swooshes by them. Kukui wonders if it was Tapu Koko, which is quickly confirmed as it turns out that the Fightinium Z on Hala's palm has been replaced with an Electrium Z. Surprisingly, Hala bursts out laughing, saying that it's extremely peculiar for Tapu Koko to take interest in a challenger like this. He wonders if there ever will be a time when he'll understand its reasons, but nevertheless, he then hands Ash the Electrium Z, which the young Trainer gladly accepts. Hala then turns to Professor Kukui, asking if he could join the planned celebration, which Kukui is happy to let him do. Ash asks if he could invite his classmates to the celebration as well, as it was thanks to them that he was able to get his Z-Crystal in the first place. Hala thinks it's a great idea, and tells Ash to feel free to invite his friends to join them.
Later that night, Ash, his classmates, Kukui, Hala, Officer Jenny, and their Pokémon all join together in a feast to celebrate Ash passing the grand trial. While the feast is going on, Ash and Pikachu walk away from the crowd. Ash, looking at his new Electrium Z shining in the moonlight, thanks Tapu Koko for watching over his grand trial, and promises to show it a successful Gigavolt Havoc the next time they battle.