No sooner have Ash and Goh arrived at Wyndon Stadium for the long-awaited World Coronation Series Masters Tournament that Hop runs up to them and demands a battle with Ash, despite being fully aware of his status as a member of the Master Class. Not one to turn down a challenge, Ash immediately accepts, but the match ends up being short-lived, as Pikachu defeats Hop's Wooloo with a single Iron Tail. To Ash and Goh's surprise, Hop reveals that he's Leon's younger brother, having sworn to be the one to defeat his big brother. Nearby, Team Rocket is hosting a live stream of their radio show to cover the tournament, although their main goal is to scout out strong Pokémon for them to steal.
After reuniting with Iris on his way to the locker room and the rest of the Master Class in said room, Ash joins them for the opening ceremony. One by one, the announcer introduces the Masters Eight to the audience: Ash, the first Champion of Alola; Iris, the aspiring Dragon Master and young Champion of Unova; Alain, a Mega Stone excavator and the winner of the Lumiose Conference; Diantha, the famous actress and Champion of Kalos; Lance, a member of the Pokémon G-Men and the newly crowned Champion of Kanto and Johto; Steven Stone, the heir of the Devon Corporation and the Champion of Hoenn; Cynthia, a renowned authority in Pokémon archeology and the Champion of Sinnoh; and Leon, the undefeated Galar Champion and the reigning Monarch. With the introductions done, the match-ups of the first round are selected. Leon will battle Alain, Lance is set to confront Diantha, Cynthia will face Iris, and Steven is pitted against Ash. While the other Champions return to the locker room to wait for their respective turns, Ash joins Goh and Hop in the stands to experience the atmosphere of the battle first-hand.
Leon and Alain enter the battlefield to begin their match. Dan recites the rules, including that the matches in the first round will be three-on-three. At his signal, the two Trainers send out their first Pokémon, with Alain leading with Chesnaught and Leon choosing Rillaboom, making the first match-up a clash between Grass types. Alain makes the first move with Hammer Arm, after which Leon surprises everyone by going for his Gigantamax right off the bat, summoning Gigantamax Rillaboom onto the battlefield. His first attack comes in the form of a G-Max Drum Solo. While the G-Max Move is able to penetrate Chesnaught's Spiky Shield, the Spiny Armor Pokémon still withstands it, like Alain had planned. Rillaboom next attacks with Max Airstream, which Chesnaught rides with Gyro Ball. Chesnaught manages to land a hit on its opponent, though backstage, Diantha notes that Rillaboom's Speed has now increased as a result of Max Airstream. After blocking Chesnaught's Pin Missile with Max Quake, Rillaboom reverts to its normal form and proceeds to use its Speed increase to outmaneuver Chesnaught's Gyro Ball and defeat it with Acrobatics, giving Leon the first knockout.
Alain's next choice is his Charizard, who easily powers through Rillaboom's Drum Beating with Flare Blitz and deals a super effective hit, taking Rillaboom out and evening the odds. Leon responds by sending out his own Charizard. However, after an exchange of attacks, Alain figures he can't get close enough and recalls Charizard in favor of Malamar, hoping to use it to turn things around. This hope is soon crushed, however, as Malamar's Psycho Cut is blocked by a Fire Blast, and the Overturning Pokémon's attempt to use Throat Chop ends with it being grabbed and taken down by a close-range Fire Blast, leaving Alain with just his Charizard, which rejoins the battle while Leon announces that he will keep his own Charizard out. Alain declares that it's his goal to surpass Leon's Charizard, and uses his Mega Ring to Mega Evolve his partner into Mega Charizard X. Alain's Charizard powers through its opponent's Ancient Power with Dragon Claw and deals a successful hit, but its subsequent Flare Blitz results in it getting struck by a point-blank super effective Dragon Pulse, defeating it and eliminating Alain from the tournament.
With Leon moving on to the semifinals, the first battle of the first round of the Masters Tournament has come to a close. Alain accepts his loss and swears to redo his training in preparation for a rematch. To decide Leon's next opponent, Lance and Diantha leave the locker room and step out onto the battlefield, preparing to start their own battle.