Lunala recovers due to the light generated by Ash, Lana, Kiawe, and Gladion's Z-Rings, but it thrashes around, wanting to go help Nebby. The Ultra Guardians are determined to go with Lunala. Despite Lusamine's apprehension, she eventually relents and the Ultra Guardians go to prepare for their new mission.
Later, at sunrise, Team Rocket wanders into the Pokémon School, determined to capture Lunala and hand it over to Giovanni before Matori does. The school's river opens up and Lunala flies out, followed by the Ultra Guardians on their Ride Pokémon. Seeing this, Team Rocket decides to chase them. Down in the Ultra Guardians' base, Lusamine, Professor Burnet, and Wicke debate over following the Ultra Guardians, but Professor Kukui volunteers to go, saying the Matori Matrix might appear again. Faba agrees with Kukui and asks Lusamine to remain at the base to gather information and give orders, while he volunteers to go with Kukui. Faba quickly declares himself the team leader and the duo depart.
Back with Lunala, it opens up and enters an Ultra Wormhole followed by the Ultra Guardians. At the base, Wicke reports that communications with the Ultra Guardians have been cut off. Meanwhile, the Matori Matrix spot the Ultra Wormhole and prepare to enter, but their helicopter is attacked by Kukui's Braviary. The helicopter sustains damage, but Matori insists they continue. Gozu overrides her commands and orders the pilots to fall back momentarily, when one of them spots Kukui, Faba, and their Pokémon on Ten Carat Hill. Kukui declares that he won't let the Matori Matrix interfere with the Ultra Guardians. As the helicopter descends, Kukui and Faba have Braviary, Alakazam, and Hypno use Rock Slide, Shadow Ball, and Psychic respectively on the helicopter, damaging it even further. Matori tells Gozu to defeat the enemies and the latter orders a counterattack.
On the other side of the Ultra Wormhole, Lunala and the Ultra Guardians arrive in a dark world. Ash notices Poipole's excitement then the Ultra Guardians are attacked by a stream of purple liquid. Kiawe has Charizard reply with Flamethrower and when the smoke clears, the attackers are revealed to be several Poipole, who resume attacking them with Venoshock. Ash tries to tell the Poipole that he and the Ultra Guardians are not the enemies and just want to save Nebby, but they don't listen. Ash's Poipole goes on to explain to its kin, and they stop their attacks. Then, Naganadel comes out of the hive and cries out. Ash's Poipole is happy to see Naganadel as Lana notes its resemblance to Poipole. Rotom notes that it has no data on Naganadel. Then, Naganadel collapses, worrying Ash's Poipole. Despite Gladion's concerns about losing track of Lunala, Ash is determined to help Naganadel and goes to its side. Lana, Kiawe, Lillie, Sophocles, and Mallow all follow suit, forcing Gladion to go with them.
Meanwhile, in Alola, Burnet tries to decipher the Altar of the Sunne mural to find the connection between the "Blinding One" and UB Black. Back with the Ultra Guardians, they check on Naganadel's condition and conclude that it's tired. Mallow offers some Sitrus Berries, but Naganadel refuses them until Ash's Poipole convinces it otherwise. Naganadel eats the Berries, and a voice thanks them. As the Ultra Guardians wonder where the voice came from, Naganadel reveals itself to have been the one who said it, and Gladion recognizes it as telepathy. Naganadel asks them if they want to save Nebby and Ash confirms it. Kiawe states that UB Black possessed Nebby and fled to this world, to which Naganadel asks if "UB: Black" is Necrozma. As Ash repeats the name, Naganadel requests for Necrozma to be left alone, stating that they need Necrozma's power. Gladion and Lillie ask for an explanation, and it uses telepathy to show the Ultra Guardians a vision of the past, back when the world was shining with light, with flowers everywhere. Naganadel explains that the world was once filled with light rained down by Necrozma. Ash sees Ultra Necrozma and realizes it is the "Blinding One". Then, the vision switches to a meteor falling to the ground, which Mallow and Lana note that if it was to hit, then it would cause a huge explosion. Necrozma collides with the meteor and destroys it, causing a bright flash of light, and when the light clears, Necrozma is revealed to have changed form and turned black; Gladion makes the connection between the "Blinding One" and UB Black.
Naganadel explains that Necrozma protected the world, but at the cost of its light, causing it to enter a deep slumber. Without Necrozma's light, the plants withered and the lands became desolate. The Poipole and Naganadel were forced to live huddled together to make use of the little light that was left. Naganadel tried to find a new place for it and its kin to live in but was unable to do so as it was too old. Then, it shows a lone Poipole flying into the distance as Rotom recognizes it as Ash's. Naganadel reveals that Ash's Poipole set off on its own as the vision ends. Naganadel continues, stating that they had given up hope on their world, when Nebby and Lunala wandered into it, causing Necrozma to awaken from its slumber to absorb their light and heal itself. Lillie notes that the vision is similar to the legend of the "Blinding One" and tells it again at Mallow's request. Mallow asks Naganadel if it wishes for Necrozma to restore the world, and Naganadel confirms it. However, Gladion notes that they will lose Nebby, and Sophocles asks what will happen to it. Ash answers that Nebby will likely stay possessed. Kiawe and Lana note that if they free Nebby, then this world will remain in its current state.
Then, a loud noise is heard as Ash and Gladion run outside to see Lunala falling to the ground, as a result of a Prismatic Laser from Dusk Mane Necrozma. Gladion rides over on Silvally to assist Lunala, only for Ash, who tries to warn him. Necrozma jumps on top of Lunala and prepares to attack it with Metal Claw, but Silvally fires an Air Slash, causing Dusk Mane Necrozma to back away from Lunala. Dusk Mane Necrozma attacks Silvally and Lunala, but they both dodge. Gladion orders Silvally to use another Air Slash, but Ash stops him. Gladion states that they cannot leave Dusk Mane Necrozma to its own devices at the risk of losing both Nebby and Lunala. Ash replies that they cannot drive Necrozma away and leave this world in its current state either, so he and his friends agree to save everyone. Ash figures that Necrozma craves Ultra Aura, almost like it was starving; and he has a flashback to when he, Lana, Kiawe, Lillie, and Mallow shared some of their lunch with Sophocles. Ash makes the same Pokémon puns he made back then to explain what they should do, to Gladion's confusion. Lana and Mallow explain the meaning of Ash's words, and Sophocles figures that they can give Necrozma their Z-Power to make it release Nebby; Gladion agrees to go with Ash's idea.
Back on Ten Carat Hill, four Team Rocket Grunts and Gozu descend to confront Kukui and Faba. One grunt attempts to fight Kukui and Faba, but Gozu steps in and sends out his Aggron. Gozu reaches into his uniform for a necklace with an attached Key Stone, and promptly Mega Evolves Aggron into Mega Aggron. Mega Aggron's Heavy Slam defeats Kukui and Faba's Pokémon in one hit, much to the two's shock. Meanwhile, Jessie, James, and Meowth watch the events unfold from their balloon. James and Meowth clarify that Necrozma possessed Nebby and fled through an Ultra Wormhole, and Lunala and the Ultra Guardians went after them. Jessie declares that they should capture them. James asks Jessie if she plans on going through the Ultra Wormhole, to which the latter replies that instead, they're going to wait for the Ultra Guardians to return to capture Nebby and Lunala.
In Ultra Guardians' base, Burnet manages to decipher the text and reveals that UB: Black is the "Blinding One" who lost its light. Furthermore, she also uncovers more details about the legend: it wasn't just Solgaleo and Lunala that shared their light with Necrozma, the people and Pokémon of Alola also shared their light with it.
Back in Poipole's world, Lunala and Dusk Mane Necrozma resume their battle as Kiawe, Lana, Ash, and Gladion all charge up their Pokemon's Z-Moves and unleash their respective Z-Powers. The four Z-Power blasts combine into a single beam of energy that hits Dusk Mane Necrozma.