Hapu is the new kahuna of Poni Island! After her confirmation ceremony, she agrees to battle Ash in a grand trial. It’s a one-on-one match between Mudsdale and Pikachu, with Kiawe as referee!
As a Ground-type Pokémon, Mudsdale is immune to Pikachu’s Electric-type attacks. On top of that, its Ability raises its defense every time it gets hit! Things are looking dire for our hero when Pikachu maneuvers Mudsdale to the edge of the battlefield—where it gets completely soaked by a wave. And since water conducts electricity, Ash and Pikachu win their fourth grand trial with a devastating Z-Move, and Ash gets a new Z-Crystal: Steelium Z!
Ash and his classmates gather at the Ruins of Hope to witness Hapu's island kahuna inauguration ceremony. As Kiawe warns everyone else to be quiet, he yells at Rotom for taking snapshots of the ceremony, only to be shushed himself by the girls for shouting. Hapu, wearing a blue dress, kneels at the ruin's altar and begins to dance for Tapu Fini. As Hapu twirls around, Tapu Fini emerges from the nearby water to witness her performance. Hapu soon removes her Z-Ring from her wrist and pledges her desire to look over Poni Island and its surrounds. Tapu Fini sprinkles the Z-Ring with some mystical water and causes several water spouts to rise from the ruin's canals to officiate Hapu as the Poni Island kahuna. Tapu Fini soon leaves the area, and Hapu's attention quickly turns to her grand trial against Ash.
Hapu walks everyone over to the Poni Breaker Coast and points out an ancient battlefield that has begun to emerge from the low tide. Hapu announces that the battlefield will be the site of Ash's grand trial against her and Mudsdale. Ash agrees to the one-on-one conditions before Hapu flies off on her Golurk. While they wait for the tide to recede, Ash and his classmates spend their time at the beach. Lillie and Mallow watch Snowy and Shaymin play in the waves. Ash's attention turns to Lycanroc as it eagerly digs in the sand to reveal something buried, and to Ash's surprise it is his Rowlet. Suddenly, Brionne splashes Ash in the face, and this quickly escalates into a water fight. As Ash steps backwards he accidentally tramples Pikachu, causing himself, Lana and her Pokémon to receive a powerful electric shock. Meanwhile, Kiawe and Sophocles lay in the shade as they discuss which Pokémon Ash will choose for his grand trial battle against Hapu.
By nightfall, the ocean is at low tide and the battlefield is now fully exposed. Ash's classmates watch from the grandstand and are surprised by Ash's choice of Pikachu because of the type disadvantage. Before the grand trial begins, Hapu recites a prayer to Tapu Fini and on her request, Kiawe assumes the role of referee and calls for the challenge to start. Hapu orders the first move, High Horsepower. Pikachu lands the first strike with a Quick Attack. While Mudsdale is stopped in its tracks, its Stamina Ability is also triggered, leading to an increase in its Defense. Pikachu lands another two Quick Attacks, and Mudsdale's defenses continue to rise. Mudsdale approaches with another High Horsepower, and Pikachu rushes in with Quick Attack. Ash has Pikachu swap to an Iron Tail, but he is sent flying from Mudsdale's Double Kick. Hapu asks Ash about his next move, and Ash replies that he always fights with full power. Ash's determination leads Hapu to step up the grand trial's stakes, ordering Mudsdale to use Bulldoze. Pikachu is knocked back by the seismic wave, which also causes a pillar to fall into the ocean and subsequently splash Ash's friends. Pikachu manages to dodge the follow-up Bulldoze attacks, but the sheer power causes some of the battlefield's surrounding pillars to collapse.
Hapu has quickly honed into Ash's evasion technique and begins to perform Tectonic Rage. The powerful Z-Move causes a giant fissure to split the ground and Pikachu falls as well. Pikachu manages to gain some footing on the debris, but Mudsdale, who is surrounded in an orange light, is on Pikachu's trail. Mudsdale soon catches up to Pikachu and sends him twirling into the air. Hapu's Pokémon emerges from the fissure before it seals shut. Meanwhile, as Pikachu begins his descent to the battlefield, Ash calls for an Electroweb to soften the landing. The electric webbing stretches over the tops of the battlefield's pillars, but the heightened tension of the threads causes Pikachu to bounce off the Electroweb and into the water. Pikachu soon swims back to the trial site unfazed. Ash considers a counterattack strategy as he thinks back to the afternoon, in which Pikachu earlier struck everyone with an electric attack. Ash orders Pikachu to bounce off the pillars with Quick Attack, and he evades Mudsdale's Stomping in the process. Ash then calls for Pikachu to stop. Hapu then sees that Pikachu is cornered and orders Mudsdale to use Bulldoze. Pikachu quickly bounds off a nearby pillar, and Mudsdale's attack causes the pillar to collapse. The resulting wake drenches Mudsdale in seawater.
Pikachu follows up with an Electroweb which ensnares and appears to be causing Mudsdale some pain. Rotom performs some analysis and reports that the seawater on Mudsdale has induced the effects of the move Soak, effectively changing Mudsdale's to a pure Water-type Pokémon. With Mudsdale now susceptible to electric attacks, Ash has Pikachu perform Gigavolt Havoc. The blinding Z-Move leaves Mudsdale unable to battle, and Kiawe declares Ash the winner of the grand trial. Hapu quickly recalls Mudsdale to its Poké Ball before she commends Ash on his unconventional strategy. She then carries over a chest filled with a complete set of elemental Z-Crystals and asks Ash to select only one. Ash asks for Pikachu's opinion, and he goes on to pick the Steelium Z. Everyone hears singing and soon notice that Tapu Fini is flying overhead. As Ash attempts to thank the island deity he trips over and falls into the ocean. Ash soon emerges from the water with his Steelium Z still in hand.
The following morning, Professor Kukui arrives to pick up his students. The class thanks Nurse Joy for caring for them, and she reports to Kukui that everyone completed their assignments. As Kukui goes to asks about their topics, each student quickly informs him of their latest ventures while on Poni Island. Kukui is rather impressed by his students' progress. Suddenly, Hapu and her Golurk land in time to see Ash and his classmates leave. She presents them with several Poni radishes and agrees to battle Ash the next time he visits the island. Hapu and Nurse Joy wave goodbye to the class as they cruise back to Melemele Island. Inside the ship's cargo hold, several Meltan have also hopped aboard, unbeknownst to anyone.