This article is about an episode of the Pokémon animated series that has not been dubbed into English. As such, its coverage may contain romanized Japanese names, rather than dub names.
Spoiler warning: this article may contain major plot or ending details.
Plot
Arriving at a train station to find Roy, Uruto finds him with the help of a passerby. To Uruto's shock, Roy is not alone, as he spots him with an unknown girl. Meanwhile, Liko and Roy, unaware of Uruto nearby, contact Dot for more information about the next Laquium sighting before heading to the site of the next Laquium sighting to begin their investigation. Once there, Crocalor and Kilowattrel search from the sky as Liko, Roy, and their remaining Pokémon search on the ground. Stopping for a brief moment, Liko reminisces the Tera Training they went through the year before as Dot wonders about Pagogo. Liko sadly informs Dot it hasn't left its Poké Ball since leaving Laqua but cheers up when Dot points out Hattrem's ease with Pagogo's Poké Ball. Just then, Roy spots some Pokémon running away and he, Captain Pikachu, and Lucario go in the opposite direction to find the terrain filled with holes. Cap feels someone watching them, and when he turns around, misses Uruto and Sableye, who fall into one of the holes. Above ground, Liko and Roy investigate the holes and find more pink mist, which Dot reveals they have been calling such proof as "Laquium Signs". Below ground, after falling down a hole, Uruto moves further into the tunnel, only to find a Steelix acting just like the Slakinghe encountered before. The Iron Snake Pokémon notices Uruto and suddenly bursts through the ground above. Liko, Roy, and Dot are surprised by Steelix and make a quick retreat when it suddenly attacks them.
Forced to battle, the two quickly find out physical attacks are not as effective due to Steelix's high Defense and that they have difficulty pinning it down thanks to the holes it made. As such, Dot has Roy send Crocalor and Kilowattrel to attack from the sky while Lucario and Meowscarada try to make an opening for Crocalor. With Lucario's Aura, it points Meowscarada to the hole where Steelix is likely to appear and she manages to hold it down when it appears. Spotting Crocalor getting ready to attack with Flamethrower, Steelix counters with an attack of its own, only for Hattrem and Cap to stop the attack. Meowscarada loses her grip on Steelix as the latter retreats into the hole once more. Roy forces Steelix to come out of hiding by having the two Pokémon in the sky use Uproar and Disarming Voice into the same hole it retreated into earlier. Once it comes out, Crocalor tries to use Flamethrower again but a sneak attack from the side sends Crocalor and Kilowattrel to the ground as another Laquium-infected Steelix reveals itself, surprising the group.
Using Sand Tomb to attack Crocalor and Kilowattrel, Liko, Roy, and Dot are further shocked by the power of Steelix's attack, attributing the cause to be the Laquium. As Cap goes to save the two Pokémon trapped in Sand Tomb, Meowscarada and Lucario are targeted by Steelix. Before one of them could successfully attack Meowscarada, she is saved by Sableye, with Uruto appearing from a hole, to Roy's surprise. Uruto notes the severity of the situation before he and Roy Mega Evolve Sableye and Lucario, respectively. The two Mega Pokémon land successive attacks on each Steelix before the two appear to be cornered. At the last second, they dodge the upcoming attack from both Steelix, causing both Laquium-infected Pokémon to hit Sableye's gem. Cap comes up with Crocalor and Kilowattrel and Roy has Crocalor use Flamethrower, this time successfully landing the attack.
Unfortunately, when the smoke clears, both Steelix are attacking each other, unable to tell each other from friend or foe due to the Laquium. Recalling what happened on Laqua, Liko desperately tries to stop them from fighting, but when attacks from both Steelix nearly hit her and her Pokémon, the three are saved by a protective barrier put up by Pagogo, who awakens from its Poké Ball. Pagogo then tranforms into its Terastal Form to purify both Steelix of the Laquium before Hattrem heals them with Heal Pulse. Roy and Dot are relieved to see Pagogo awake while Liko happily greets Pagogo. Roy then introduces Uruto to Liko, who thanks him for saving Meowscarada earlier. After stopping Sableye from eating Pagogo, Liko asks Roy and Uruto about Mega Evolution, which Nidothing explains in a video. Uruto chastises Roy for abandoning him in the forest but Roy realizes Uruto had been following him, which he denies before declaring the Black Rayquaza would be his. Roy drags Liko away during Uruto's monologue, not desiring to bring him along their journey, to Uruto's frustration at being left behind once more.
At the Explorers base, Hamber meets with Zirc and Onia. The two agents lament their inability to protect their boss, who is now missing, and are prepared to take accountability for their actions. To their surprise, Hamber does not reprimand them. Hamber reveals to the two that after the events in Laqua, Spinel made a report claiming that Amethio had betrayed Gibeon and that he had also perished, but neither him nor his father Cervantis believed the report. He then takes out a Zygarde Cube and the lone Zygarde Cell, demonstrating how the cube works. Hamber assigns them a new task - to find the rest of the White Zygarde and Amethio without letting Spinel know what they were up to before it's too late.
For the initial airing of this episode, the Mega Nidothing's Mega Mega! Mega Evolution segment was absent and the episode was followed by a segment featuring Professor Willow, announcing an event in Pokémon GO featuring characters from Pokémon Horizons: The Series. This trailer was later uploaded to the Japanese Pokémon YouTube channel.
