Our heroes are all gathered at the Altar of the Sunne, along with all four of Alola’s Island Guardians. Lillie and Gladion ask the Island Guardians for help to rescue their mother, Lusamine, who has been taken through an Ultra Wormhole by a mysterious Ultra Beast. After Tapu Koko and the others perform a ritual, little Nebby evolves into the mighty Solgaleo—one of the Legendary Pokémon of Alola!
At Solgaleo’s invitation, the Pokémon School students climb onto its back. With the help of a new Z-Move, Solgaleo opens an Ultra Wormhole and carries them through, off to rescue Lusamine from the Ultra Beast!
Lillie recaps the events of the last episode, in which she and Gladion made their way to Vast Poni Canyon in an effort to save their mother Lusamine. They successfully battled against Kommo-o and went on to reach the Altar of the Sunne. Her friends, including Ash and Professor Kukui, were in tow, much to Lillie's delight, concluding the flashback.
With everyone assembled, they notice Alola's four guardian deities hovering above them. Pikachu first approaches Tapu Koko, and Ash explains the situation to them. Lillie elaborates that Nebby's power to open an Ultra Wormhole would help save Lusamine, and Gladion asks if it was possible to meet Solgaleo at the Altar of the Sunne. Immediately, Tapu Koko flies over to the other guardian deities, and after a brief discussion, the quartet sends out a powerful cry and a multicolored sphere appears. Rotom explains that the island deities' Psychic, Electric, Misty, and Grassy Terrains have mixed. A beam of light descends and forms a viewing screen. Solgaleo, followed by Lunala, emerge from separate Ultra Wormholes and clash, just as in Ash's dream earlier. A Cosmog then emerges from the cloud, which Mallow recognizes as Nebby.
Nebby instantly teleports the four island guardians to a location near the Pokémon School. It is revealed that Tapu Koko lured Ash and Pikachu to Nebby. The viewing screen suddenly fades. Professor Kukui tells Ash that he believes that the guardian deities, Lunala, and Solgaleo entrusted him with Nebby. Whilst Rowlet is fast asleep inside Ash's bag, something begins to glow; Rowlet hurriedly leaves Ash's bag, and Nebby floats out. Ash shows Tapu Koko the peculiar cocooned Pokémon, which soon continues its ascent into the sky. Tapu Lele and Tapu Fini begin to throw glitter and water vapor around Nebby. Gladion becomes infuriated by the delay, though Tapu Koko silences his concerns with an intense glare. Lillie reassures her older brother that the island guardians are capable and require everyone's patience. Kukui leaves the group after receiving a phone call, explaining that Professor Burnet was due to arrive. As he departs, he whispers to his Braviary still in its Poké Ball.
Meanwhile, the Team Rocket are climbing their way to the Altar of the Sunne after a tough match against several Jangmo-o. Upon making it there, James pulls out a book, and reveals the mysterious floating Pokémon are Alola's guardian deities, though his reference doesn't contain Cosmoem. Meowth overhears Ash and his friends' conversation, which reveals that Nebby has since evolved. The trio are still under the assumption that Nebby is a pre-evolved form of Koffing, and decide to catch it and the guardians.
Meanwhile, Burnet and her Munchlax are surrounded by a horde of Jangmo-o and Hakamo-o when Kukui arrives on the scene. He calls upon his Braviary and flies over to Burnet's side. The obstacle is cleared after one Brave Bird attack, much to Burnet's surprise.
Elsewhere, Team Rocket is admiring their latest targets through a set of binoculars. Suddenly, Bewear arrives and carries Team Rocket away, jumping across to another cliff. Bewear holds the group tighter, causing them pain before releasing them. Team Rocket watch the unfolding scene in awe.
Now, the four island guardians are hovering above their respective pillars and humming in harmony. The ground begins to glow, and a light traces its way upwards and opens the Altar of the Sunne. The now opened portal beams a light directly onto Nebby, and in a light and fire spectacle, it emerges as a newly evolved Solgaleo. Rotom takes a flurry of images, and records that Cosmoem evolved into Solgaleo. Jessie notes that the evolution is cool, before she and Meowth chastise James for his wrongful claim that Nebby was an Alolan pre-evolved form of Koffing. As Ash and his friends are in awe of Solgaleo's presence, Lillie approaches Solgaleo before embracing it. Ash offers Solgaleo some candy, which it happily accepts and licks Ash with its giant yellow tongue as thanks.
Ash, Lillie and Gladion then ask Solgaleo if it can take them through the Ultra Wormhole. A Z-Crystal appears, landing in Ash's hand. The peculiarly-shaped Z-Crystal does not fit into Ash's Z-Ring, with Tapu Koko removing it from his wrist. It closes its shell with the Z-Ring inside and performs a ritual chant and dance before passing it on to Tapu Lele, followed by Tapu Bulu and Tapu Fini, all of them upgrading Ash's Z-Ring. Ash thanks the island guardians for the upgrade, which can now fit the Solganium Z. Gladion immediately requests Solgaleo to take him through the Ultra Wormhole to rescue his mother. It appears a little startled by the forward request, before it lays flat to allow everyone to climb aboard. Kiawe commands the group to return their Pokémon to their Poké Balls before they depart.
With everyone on board, Solgaleo stands up. Tapu Koko floats over and shows Ash the Z-Move pose in several stages. A sun symbol emerges on Solgaleo's forehead, and with a roar, it becomes a bright light and performs Searing Sunraze Smash, creating an Ultra Wormhole opening for the group to float through. Team Rocket is astounded and believes rare treasures and Pokemon may be behind the portal. Out of curiosity, Jessie asks Bewear if it could jump through the portal, but it holds the trio tighter to inflict pain. Kukui and Burnet arrive at the Altar in time to see the opening closing and the guardian deities departing. Solgaleo runs through the Ultra Wormhole, while everyone holds on tightly.