At the Shalour City Gym, Ash and Goh observe Korrina Mega Evolving her Lucario. Korrina's challenger has his Quilladin attack with Wood Hammer, but Mega Lucario's Aura Sphere immediately overpowers it, defeating the Spiny Armor Pokémon and making Korrina the winner. With the battle over, Ash and Goh greet Korrina and descend down to the battlefield. Gurkinn also greets them and congratulates Ash for advancing to the Ultra Class in the World Coronation Series. Ash explains to Korrina that he wants to Mega Evolve his Lucario too in preparation for his upcoming Ultra and Master Class battles. Remembering Ash's Riolu's fascination towards her Lucario, Korrina gladly agrees to help.
Ash, Goh, Korrina, and Gurkinn move to the top of the Tower of Mastery, where Korrina hands Ash a pair of Mega Gloves, with a Key Stone embedded in one of them. Ash puts them on and admires his new Key Stone sparkling in the sunlight. Ash's Lucario suddenly becomes surrounded in an Aura, which Goh mistakes for Lucario being about to Mega Evolve. Gurkinn corrects him on this, explaining that Mega Evolution requires the Trainer to hold a Key Stone and the Pokémon to hold a Mega Stone. Each Pokémon capable of Mega Evolving has its own Mega Stone, with Lucario's Mega Stone being called the Lucarionite. Lucario walks to the edge of the balcony and points towards the sea. Gurkinn says that Ash's Key Stone has enhanced his Lucario's Aura, and Korrina realizes that it's pointing towards Mega Island.
The group gets on a boat, where Gurkinn explains that Lucario had sensed the location of its Mega Stone. As they approach Mega Island, Korrina and Gurkinn explain that it suddenly appeared one day and was given its name due to the large number of Mega Stones found on it. Because of this, many Pokémon capable of Mega Evolving are also present on the island, making Goh excited. When they make land, Gurkinn stays at the boat, while Ash, Goh, and Korrina move on to go find a Lucarionite. They soon spot a shine inside a cave and run after it. They soon realize, however, that the shine was coming from the eyes of a Sableye instead of a Lucarionite, just in time to slip on a frozen hillside and start sliding down towards a ravine. Lucario manages to grab the edge of the cliff with the spikes on the back of its hands and grab Ash with its legs, while Ash grabs onto Goh and Goh grabs onto Korrina. Pikachu retrieves Dragonite and Flygon's Poké Balls and calls them out to pick up the group. While flying to safety, the group notices an Abomasnow on the other side of the ravine and realizes that it was the one who had frozen the hillside.
The group continues to move further into the island. Just as Ash asks Lucario if it can sense where exactly its Mega Stone is, they notice a Salamence and a Pidgeot battling in the air above them. Pidgeot's Gust causes everyone to fall off Dragonite and Flygon and onto the giant lily pads below. The giant leaves bounce Ash and Korrina up and down, while Goh and Lucario are sent bouncing further and further away from one lily pad to another, with Flygon following them. Deciding that flying is too risky, Ash recalls Dragonite before he and Korrina head after Goh and Lucario.
Goh and Lucario, meanwhile, land in the middle of a rocky area filled with erupting pillars of lava. While recalling Flygon, Goh notices that Lucario is having trouble withstanding the fiery environment due to being a Steel type. To solve the problem, he calls out Panpour and has it use Rain Dance, creating a rain cloud over them as they try to get out of the area. On their way out, Goh spots a Camerupt in their way and catches it, and has Cinderace knock down a lava pillar with Pyro Ball. They eventually make it to a cooler environment, where Goh thanks his Pokémon for their help before recalling them.
Goh attemps to call Ash with his Rotom Phone, but he gets no signal. As such, Lucario starts using its Aura to contact him instead. This causes a reaction in Ash's Key Stone, so he closes his eyes and focuses to sense Lucario. He and Lucario manage to sense each other's Aura, with Ash also sensing Goh's presence near Lucario. However, when Lucario starts heading towards Ash, it senses something else in another direction and heads that way instead. Ash senses this too and understands that Lucario has located its Mega Stone, so he and Korrina head towards where Lucario and Goh are headed.
The two pairs soon find themselves and each other in the midst of an area filled with tall metal spikes rising from the ground, which Korrina tells is called the Steel Forest. Lucario stands before one of the spikes, sensing that the Lucarionite is located inside it. As such, Ash tells Lucario to use Aura Sphere on it. The impact sends a shockwave through the metal spike, shattering it and revealing a Lucarionite, which strangely floats in mid-air. A laughter fills the air, and an old man referring to himself as the Mega Evolution Sage appears before them, floating in the air as well thanks to his Alakazam. He is lowered to the ground and declares that if Ash wishes to get the Lucarionite, he'll have to get past him first. Despite Goh being displeased by this unexpected obstacle, Ash accepts the challenge.
The Mega Evolution Sage starts the battle by Mega Evolving his Alakazam and having it bring the Lucarionite behind it. Lucario makes the first move with Aura Sphere, which Alakazam counters with Psybeam. Lucario then uses Double Team and charges at Alakazam, who uses Psychic to fire its spoons at the copies and locate the real Lucario amongst them, before following up with Tri Attack, which knocks Lucario down to the ground and inflicts it with paralysis. Regardless, Ash tells Lucario to jump up using the metal spikes. However, the paralysis catches up to Lucario mid-jump, causing it to fall down again. Ash encourages Lucario to try again, and while the paralysis hinders this attempt as well, Lucario manages to leap over Alakazam by grabbing on its beard, successfully grabbing the Lucarionite.
While Ash and Lucario celebrate their success and Goh and Korrina congratulate them, the Mega Evolution Sage laughs and pulls off his outfit and beard, revealing himself to have been a disguised Gurkinn all along. Ash and Goh are even more surprised when they find out that Korrina knew it was him the whole time. As everyone boards the boat and heads back to the mainland, Ash receives a notification on his Rotom Phone, learning that his next World Coronation Series opponent has been decided, and it's none other than Bea. Ash and Lucario smile confidently at each other, ready to win their third battle against their rival.