At Mount Lanakila in Alola, Quillon and Danika depart after returning the Alolan Ninetales rescued by Goh to its natural habitat. Quillon mentions that the Pokémon Hunters who originally stole it have been located and they're heading their way next. Danika asks if he's curious about the Trainer who caught Ninetales, as he's about to start his first Trial Mission. Quillon feigns disinterest, but Danika can easily tell he is curious.
At the Cerise Laboratory in Kanto, Goh is excited to receive a notification inviting him for his first Project Mew Trial Mission. Of the three options he's given, he chooses the one that requires finding "Volcarona's golden scales". He and Ash are confused, since Volcarona's scales shouldn't be gold-colored, but Professor Cerise figures that figuring out that part is a part of their mission. Looking at his Rotom Phone, Goh figures that the best place to start their search would be the Colossus Ruins in Unova region's Desert Resort, the same place where he caught his Golurk. They remember the golden room that was in the ruins, and figure that could be where they could find a Volcarona with golden scales.
Ash and Goh arrive at the Colossus Ruins, where they meet the excavation leader and ask him about Volcarona. However, the leader mentions that the Volcarona sightings stopped a few days ago after Pokémon Hunters had started capturing Pokémon nearby. He does, however, have a Volcarona-related discovery to show to them, and leads them to a mural in the golden room, featuring a Volcarona. The leader translates the ancient text on the mural, describing how Volcarona saved those suffering from the cold when the "mountain's rage" covered the sky. Goh figures that "mountain's rage" could mean a volcanic eruption, which means that Volcarona could perhaps be found in a volcanic area. The excavation leader mentions that past the ruins, there's a town founded during a gold rush, which was sealed off after a volcano erupted, and suggests they search from there.
Arriving at the town, Ash and Goh find it to be a dilapidated ghost town. They enter an abandoned gold mine and start exploring it. They come across a Roggenrola, which Goh catches after using Inteleon's Snipe Shot to weaken it. A big horde of Roggenrola then appears and starts attacking them, forcing them to use a minecart to escape. The wild ride on the cart, during which Inteleon narrowly saves them from falling of a cliff, eventually leads them to an elevator, which Pikachu powers up with Thunderbolt. The elevator brings them to a big open space, where they find a man being attacked by a Ferrothorn. Goh uses his Cinderace to quickly defeat Ferrothorn with Pyro Ball, and then catches it. The man, Nito, thanks them for saving him and says that he's here to study the local wild Pokémon. Ash and Goh introduce themselves and mention that they're looking for Volcarona. Hearing this, Nito says that he's looking for Volcarona too, and decides to tag along.
Ash, Goh, and Nito move on and arrive at a mine railway station, which connected the mine to the next one over during the gold rush. Ash spots a group of Eggs, which Nito identifies as Eggs of Larvesta, Volcarona's pre-evolved form. Goh figures since the Eggs are here, Volcarona must be nearby to protect them. Nito then declares that he'll take the Eggs before Volcarona can come back, sending out a Galvantula and having it use Sticky Web on Ash and Goh, binding them in place. Goh realizes that Nito is actually a Pokémon Hunter, and Nito admits that Volcarona managed to get away from him when he was capturing the local Pokémon earlier, but now he intends to sell these Eggs for a profit. He piles the Eggs onto a wheelbarrow and decides to head further into the tunnels in case there are more more Eggs in there. Before he can do so, however, a pair of Larvesta emerges from the tunnel and starts attacking Nito with fire, with a third attack coming from the depths of the tunnel. Overwhelmed, Nito decides to give up on the Larvesta and makes a run for it. The Sticky Web binding Ash and Goh ends up catching fire and breaking apart, freeing them. Ash retrieves the Eggs and drives Nito off before returning them to the Larvesta, calming them down.
However, the railway station is still on fire following the Larvesta's attacks. Goh calls out Inteleon and Krabby to fight the fire, while Ash does the same with Dracovish, but their Water-type moves do little against the blazing inferno. A burning tower almost falls on top of the Eggs, but they're saved by the same flame that came out of the tunnel earlier. The flame's user emerges from the tunnel, revealing itself to be a Shiny Larvesta. It then promptly evolves into a golden Volcarona and uses Fire Spin to absorb the flames, putting out the fire. After doing so, Volcarona starts flying above Ash and Goh, scattering golden scales. Goh uses a glass jar to collect some of the scales, happy to have achieved the objective of his Trial Mission.
Ash and Goh make their way out of the mine, but once they arrive at the entrance, they're surprised to see Nito, who had secretly stayed behind to witness the golden Volcarona, and is determined to catch it, revealing that he even called up some henchmen to help him out. They send out their Pokémon and demand the two boys hand over their Pokémon. Just then, however, Danika and Quillon appear to take care of the Hunters. Nito and his henchmen attack, but before their attacks can land, both Danika and Quillon send out an Urshifu each, with Danika's Urshifu defeating most of the Hunters' Pokémon with Surging Strikes and Quillon's Urshifu taking down a Conkeldurr with a Wicked Blow. Ash and Goh are surprised to see two Urshifu, with Goh also noting how Danika's Urshifu looks different from Quillon's Single Strike Style one. His Rotom Phone informs him that Danika's Urshifu is of the Rapid Strike Style.
With the Hunters dealt with, Danika verifies the scales Goh collected as authentic, declaring that he's cleared the mission and sending him his first token. Goh feels sorry for needing to be rescued, but Quillon says they only dealt with the Hunters, and Goh didn't need any help. Professor Amaranth enters the scene as well, saying that they'll make sure the Hunters are brought to Officer Jenny and their Pokémon are eventually released back to the wild. Danika encourages Goh to prepare for the next time and not worry about anything, which he happily agrees to.