Battling the Beast Within! (Japanese: 風を断つ稲妻！その名はゼラオラ！！The Lightning Bolt that Severs the Wind! Its Name is Zeraora!!) is the 100th episode of the Sun & Moon series, and the 1,039th episode of the Pokémon anime. It first aired in Japan on December 9, 2018 and in the United States on March 30, 2019.
While battling Tapu Koko, Ash and Pikachu suddenly find themselves in an unfamiliar place inhabited by a menacing Ultra Beast known as Guzzlord! Guzzlord’s power is fearsome, but a mysterious man named Dia—accompanied by the Mythical Pokémon Zeraora—comes to our heroes’ aid. Ash wants to help Dia reclaim his home from Guzzlord’s destructive wrath, but after Dia defeats Ash in a test battle, our hero relents. He and Pikachu must return to their own home, but how? Dia offers them shelter for the night, but on the way, they discover an abandoned Pokémon School! What could all of this mean?
Ash wakes up and finds Professor Kukui, Professor Burnet, and her Munchlax outside performing yoga on their surfboards. Burnet explains that it has been a while since the pair recharged. Kukui asks Ash to look at the beautiful scenery and consider the idea that it only exists because of a balance between people, nature, and Pokémon. Ash promptly joins Kukui and Burnet in their meditation session. As the group performs some deep breathing exercises, Tapu Koko watches on from the shore.
Later, Ash rushes to the Pokémon School so he could have a battle against Kiawe. Rotom warns Ash that he won't have any energy if he continues running. Tapu Koko suddenly appears, and a few moments pass before Ash asks if it would like to battle him in preparation for his upcoming Poni Islandgrand trial. Oddly, the Pokémon remains silent and flies into the nearby forest. Following Pikachu's insistence, the group follows Tapu Koko deep into the Melemele Island forest. The pursuit ends in a clearing as Tapu Koko officiates the challenge and transforms the area with Electric Terrain. Ash happily accepts the conditions and has Pikachu rush towards Tapu Koko with Quick Attack. However, Tapu Koko dodges the attack and retaliates with Dazzling Gleam. Pikachu evades the move and uses Thunderbolt. Tapu Koko counters the attack with its Discharge. As the two Electric attacks collide into a large mass of electricity, a Shiny Tapu Koko in another dimension uses its own attack. The simultaneous attacks result in a rift which soon transports Ash and Pikachu to an unknown place while Rotom and Ash's bag remain behind.
Ash and Pikachu wake up in a desert landscape with scattered rubble throughout. Ash calls out to Rotom and Tapu Koko and gets no response. He pulls Pikachu's face only to receive a Thunderbolt, and Ash soon realizes that they are not in a dream. Pikachu and Ash soon decide to investigate the rumbling noises nearby and enter a dilapidated building. The pair narrowly avoid being hit by a large piece of debris and soon spot a mysterious large Pokémon, Guzzlord. Ash walks over, hoping to get a closer look. In the process, he accidentally kicks a rock which makes a sound that alerts Guzzlord to the pair's presence. Guzzlord smashes its tail, then turns around and lets out a frightening cry. Ash and Pikachu watch in shock as Guzzlord consumes a large piece of debris, but are soon forced to run as Guzzlord chases them. Ash and Pikachu make their way up onto another level and reach a dead end as Guzzlord smashes its way through to reach them. Ash has Pikachu unleash a Thunderbolt, but to his surprise Guzzlord simply eats the attack. With this mouthful, Guzzlord is able to use Belch at the pair, before following up with Gastro Acid, which they barely avoid. Ash tries to use his Z-Ring, but it doesn’t seem to work. Guzzlord continues to approach Ash and Pikachu, readying its claw hand to grab them.
Suddenly, a Trainer arrives on the scene and has Zeraora use Close Combat to smack Guzzlord's claw out of the way. Pikachu and Ash look up to see Zeraora as it returns to the ground. Zeraora follows up with Plasma Fists, which temporarily paralyzes Guzzlord, thus allowing Ash, Pikachu, the Trainer, and Zeraora to flee. The Trainer leads everyone down a ventilation pipe, and demands that Ash and Pikachu keep quiet to remain undetected by Guzzlord. The group soon arrives at a safe area, with Zeraora giving the all-clear. Ash formally introduces himself, and his rescuer introduces himself as Dia and his Pokémon as Zeraora. Ash turns his attention to Zeraora and adds that he was impressed by its move, Plasma Fists. Zeraora remains stoic and turns it back on Ash to avoid further questions. Instead Ash notices Zeraora's tail and goes onto grab it, only to receive a warning Thunderbolt in return. Dia demands to know how Ash reached the restricted area. Ash replies that he was simply battling Tapu Koko when he and Pikachu found themselves faced with Guzzlord. Dia adds that Guzzlord is an Ultra Beast.
In a flashback, Dia explains that Guzzlord appeared one day and began to eat everything in sight reducing the once thriving city to the Ultra Ruin of today. An Ultra Beast fighting squad called the Ultra Rangers attempted to fight Guzzlord, but their efforts proved useless. Afterwards, the Ultra Rangers and the land's citizens abandoned the area, while only Dia and Zeraora remained. Dia explains that he remained because he still remembers when the area was thriving with people and Pokémon. Ash offers to assist, but Dia says that Ash will just hold him back. Ash explains that he is part of the Ultra Guardians, but Dia says he hasn't heard of them before and dismisses the organization as a child's game. Infuriated by Dia's criticisms, Ash demands a battle to prove himself and Dia despondently agrees to the match.
Dia requests that Ash make the first move. Ash obliges and has Pikachu perform Quick Attack. Zeraora replies with its own Quick Attack. As the two attacks clash a bolt of lightning rises into the sky and neither side seems injured. Pikachu tries an Iron Tail, but the attack is overwhelmed by Zeraora's Close Combat. Ash calls for an Electroweb, and it ensnares Zeraora. As Pikachu is about to unleash a Thunderbolt, Zeraora Slashes its way out of the web and follows up with Plasma Fists. Despite Ash's warning, Pikachu is unable to leap out of harm's way and is defeated.
Pikachu has since recovered, but Dia demands that Ash return home. Ash concedes and asks how he is supposed to get home. Dia replies that he'll organize the Ultra Rangers to pick Ash up the next day. Ash is surprised and then learns that Dia is a former member of the group. Dia invites Ash over to his base, noting that Guzzlord makes camping outside too dangerous. As the group make their way to the base, Ash and Pikachu's stomachs begin to grumble from hunger. To keep them quiet, Dia throws Ash some emergency rations to feed Pikachu. Soon, the group arrives at Dia's base, but Ash is perplexed by the ruined building's remarkable resemblance to the Pokémon School.