A Dream Encounter! (Japanese: サトシとほしぐも！不思議な出会い！！Satoshi and Hoshigumo! Mysterious Encounter!!) is the 44th episode of the Sun & Moon series, and the 983rd episode of the Pokémon anime. It first aired in Japan on October 5, 2017, in New Zealand on March 1, 2018, in Canada on March 10, 2018, and in the United States on March 24, 2018 as the first episode of the twenty-first season, Pokémon the Series: Sun & Moon—Ultra Adventures.
After dreaming that he made a promise to the Legendary Pokémon Solgaleo and Lunala, Ash has a hard time waking up. On his way to school, he’s sidetracked by Tapu Koko, who guides him to a strange little Pokémon that looks like a cloud of stars. Lillie nicknames it Nebby, and Ash remembers his dream: he promised to take care of it!
This attracts the attention of Lillie’s mother, Lusamine, the head of the Aether Foundation. She and her colleagues come to meet Nebby and offer to take care of (and research) this unknown Pokémon. But Ash is resolute in his promise, so Nebby’s identity is a mystery for now!
Ash and Pikachu find themselves in mountains surrounded by fog. They watch, as in the sky, two lights appear and Solgaleo and Lunala emerge. The two Pokémon looked down at Ash who understands and makes a promise. They disappear before emitting a beam of light where Nebby is formed. The guardian deities sense Nebby's arrival as Gladion tries to calm his Silvally down. Gladion soon realizes that an Ultra Beast has appeared. At Professor Kukui's house, Rotom is trying to wake Ash and Pikachu up but they're still sleeping. With help from Litten, Ash and Pikachu eventually get up. Ash starts to remember the dream he had where he had made a promise to Solgaleo and Lunala.
During breakfast, they watch as the "Alola Woman of the Year" award is given to Professor Burnet. Professor Kukui explains that her research is best known on Ultra Wormholes. At the Altar of the Sunne, Professor Burnet, Lusamine, Wicke and Faba are examining the ruins where Professor Burnet had discovered a spike in energy readings from an Ultra Beast. Faba suspects this might have been an equipment malfunction which is put down to jealousy. They head off to do more research. Ash, Pikachu, and Rotom are on their way to the Pokémon School when they come across Tapu Koko who leads them to Nebby, sleeping in the forest. Ash recognizes Nebby from his dream and remembers his promise. Rotom has no data on Nebby.
At school, Professor Kukui greets the class and notices that Ash has yet to arrive. Ash arrives at that moment and shows Professor Kukui and his classmates Nebby. They watch Nebby and agree to do what they can to research it as Lillie gives it the nickname Nebby. Nebby starts crying leading everyone to attempt to calm it down and Mallow soon calms it down. They try and find out what Nebby likes to eat and discover that Nebby likes star candy. At Aether Paradise, Lusamine is alerted to a detection of an Ultra Beast's aura and they head to Melemele Island. Ash and Lillie arrive back at Professor Kukui's house and learn they have guests who want to see them and Nebby.
Inside, Lusamine greets Lillie and embarrasses her by treating her like a baby. Lillie isn't pleased to see her and is annoyed at her evolving Clefairy into Clefable. Professor Kukui introduces Ash to Professor Burnet, Faba and then Lusamine asks to be introduced to Nebby. Professor Burnet brings up images of the Guardian deities and Ultra Beasts and explains the legend surrounding them. They believe Nebby to be an Ultra Beast. They detected a spike in the aura's power and brings up an image of the Altar of the Sunne which Ash recognizes from his dream and explains what happened. Faba doesn't believe this as Lusamine asks Ash to let them look after Nebby, but remembering his promise, Ash wants to look after Nebby himself. Faba tries to prevent Ash from looking after Nebby but Lillie and Professor Kukui explain Ash's past with Tapu Koko and Lusamine agrees to let him look after Nebby.