Outside the Pokémon School, Ash and Pikachu are busy practicing their Steel-type Z-Move Corkscrew Crash, which allows Pikachu to easily drill his way through a series of wooden obstacles set up for him. Both Ash and his classmates are excited, with Sophocles and Mallow wanting to try out one too. Just then, Samson Oak exits the school building and reminds everyone of the important announcement that Professor Kukui is due to make at Aether Paradise.
At Aether Paradise, a crowd and members of press have gathered, with Professor Burnet welcoming Ash and his classmates. The press conference begins, during which Professor Kukui announces the first-ever Alolan Pokémon League, in which the winner of the tournament will become Alola's first-ever Champion. The press conference is broadcast live across the region and observed by people all over Alola, including Gladion and Team Rocket. The trio starts dreaming about the possibilities achievable through winning a championship, like giving the entire archipelago as a gift to the Boss.
Back at Aether Paradise, Kukui presents the crowd with Island Kahunas Hala, Nanu, Olivia, and Hapu, as well as Lusamine of the Aether Foundation, who have agreed to support his idea. The Kahunas mention the custom of island challenge and how it once included an abandoned tradition in which those who cleared all four grand trials must climb to the summit of Mount Lanakila and battle each other there, before explaining that Professor Kukui decided to revive that tradition in a form that anyone can participate in. Lusamine reveals that the Aether Foundation is building the Pokémon League stadium a short distance away from Aether Paradise.
After the press conference ends, both Ash and Kiawe immediately decide to join the League. When Principal Oak asks the rest of Ash's classmates if they want to join too, Lana is quick to accept, and Sophocles also shows interest due to his newly evolved Vikavolt. Mallow also encourages Lillie to join, reminding her of the successful Double Battle she had a while back, though both girls are uncertain about participating.
At Po Town in Ula'ula Island, Team Skull Grunts are spending time at the Shady House when Plumeria enters and announces the return of their boss, Guzma, who steps in through the front door, much to the Grunts' excitement. While Guzma asks everyone how they are doing, Tupp, Rapp, and Zipp rush in, informing Guzma about the announcement of the Pokémon League. They turn on a TV and go to a report about Kukui's League announcement. The same report is also being watched by Ash, Kukui, and Burnet at Kukui's house on Melemele Island. Ash asks Kukui if he thinks the Masked Royal is going to participate in the League, and Kukui hastily explains that because the Masked Royal is a mysterious character, he can't be sure. Regardless of that, Ash swears to start training for the League the next day. At the Shady House, Team Skull observes the interviews for each of the Island Kahunas regarding their thoughts about the Pokémon League. Plumeria asks Guzma what he wants to do, and he declares that he is going to destroy the Pokémon League. He then tells his minions that they are going to Melemele Island.
The next day, Ash and his team begin running along the beach as part of their training. Meanwhile, in Hau'oli City, Team Rocket notices a poster about the Pokémon League and decides to start training for it as well. Just then, Tupp, Rapp, and Zipp arrive, mocking the trio and performing their introduction rap. Not wanting to be outdone, Team Rocket performs a rap version of their own motto, leaving James and Meowth momentarily exhausted. While both trios prepare to battle, Guzma steps in and says that he will deal with the Team Rocket trio himself, saying that it will be a good warm-up for Kukui. He tells them to come at him all at once, swearing to destroy them. Soon enough, Team Skull exits the scene, while a defeated Team Rocket lies on the ground in their wake. Bewear then emerges from a nearby concrete pipe, scoops up Team Rocket, and runs away.
At the Pokémon School, Kukui is in the middle of explaining the Pokémon League to his class when Team Skull starts calling to him from the schoolyard, telling him to come talk to them. He goes down and tells them to save their talk for later, but his attitude changes when he recognizes Guzma. Ash and his classmates follow Kukui to the schoolyard, and Plumeria tells them they can witness the match between Kukui and Guzma. Guzma emphasizes that he is not going to battle Kukui, but rather destroy him. He goes on to call the island challenge and Island Kahunas outdated customs and adds that a Pokémon League is just as unnecessary, as he already considers himself the strongest Trainer around. Kukui asks why he won't just join the League to prove his strength, but Guzma shoots down the idea. He swears to destroy the League if Kukui doesn't back down. Kukui refuses, noting that Guzma hasn't changed at all. Ash then steps in and asks Guzma to have a battle with him. Guzma notices Ash's Z-Power Ring, and Ash tells him that he has cleared all four grand trials. Ash instantly reminds Guzma of Kukui, so he accepts his challenge. Kukui tries to talk to Ash, but he asks Kukui to leave this to him, which he agrees to, saying that he will act as the referee. Guzma swears to destroy Ash, but Kukui tells Guzma not to underestimate him.
As the two prepare to battle, Ash chooses Pikachu, while Guzma sends out Golisopod. Rotom scans Golisopod and gives Ash its Pokédex entry, noting that Pikachu has the type advantage over his Bug/Water-type opponent. However, once Kukui starts the battle, Guzma immediately calls for a First Impression, dealing the first blow on Pikachu. Ash has Pikachu use Iron Tail, but the attack is ineffective against Golisopod's diamond-hard armor. Guzma orders a Pin Missile, which Pikachu counters with Electroweb before using Thunderbolt. However, Golisopod deflects the normally super-effective move, much to Ash and Rotom's surprise. Ash decides to put the results of his training to use and performs Corkscrew Crash with Pikachu. However, Golisopod blocks and deflects the Z-Move with its armor-plated hands, surprising everyone. Guzma then orders Golisopod to use Liquidation, striking Pikachu while he is still reorienting himself. Ash has Pikachu use Quick Attack, but Golisopod retaliates with Poison Jab, poisoning Pikachu in the process. Pikachu attacks with Iron Tail, but gets stopped in his tracks by a volley of Pin Missiles from Golisopod. Despite the poison and the numerous hits, Pikachu is still able to stand up. Ash orders a Quick Attack, and this time, Pikachu dodges the incoming Poison Jab and strike Golisopod's unarmored underside, causing the tall Bug type to flinch.
Although Pikachu is getting weaker as the battle wages on, he still refuses to stay down, much to Guzma's irritation. Pikachu and Golisopod stare at each other for a moment, until Golisopod suddenly turns and uses its Emergency Exit to return to its Poké Ball, surprising Ash and Pikachu. Guzma says that Golisopod deemed Ash so weak that it lost all interest, irritating Ash. Kukui asks if Guzma is throwing the match, and he responds that he has no interest in continuing a battle with an obvious outcome. Ash protests that he can't say that for sure unless he finishes the battle, but Guzma dismisses him. Ash's attention then turns to Pikachu, who collapses from the effects of poison. Kiawe tosses Ash a Pecha Berry, which he gives to Pikachu to heal his poison. Plumeria tells Ash that he should join Team Skull, but he rejects the suggestion without a second of hesitation. Guzma declares that he will destroy the Pokémon League while all of Alola is watching, but Kukui then steps in and asks Guzma if he's going to just run away again, just like he did with the island challenge. Guzma and Kukui glare at each other for a while, until Guzma says that he almost let himself be thrown off again, and that Ash made him change his mind about destroying Kukui. He and the rest of Team Skull then turn around and walk away.
Later, back at the Shady House, Tupp asks Plumeria where Guzma is, and she replies that he's upstairs in his room making plans for the destruction of the upcoming Pokemon League. Upstairs, Guzma recalls Kukui's words about running away, and punches the wall in front of him in frustration.