Information regarding the sighting of an unknown Pokémon at Galar's Crown Tundra has reached the Cerise Laboratory, where Professor Cerise informs Ash and Goh of this. They invite Chloe to come along to investigate, which she gladly agrees to do. Meanwhile, a ship is sailing through ice-filled waters as Ash's Alolan classmate Lillie, her brother Gladion, and her mother Lusamine continue their search for her father and Lusamine's husband, Mohn. Lillie's piano playing is interrupted when Magearna once again releases a beam of light towards Mohn's location, which Gladion realizes is the same location his calculations had pointed towards: the Crown Tundra.
Arriving in the small town of Freezington in the Crown Tundra, Ash, Goh, and Chloe interview the local mayor for information about the unknown Pokémon he saw. However, the mayor is unable to tell anything but that the Pokémon was white and yellow, as he sighted it in the middle of a blizzard. As they speak, another sudden blizzard appears outside, so the mayor invites the three to spend the night here before continuing on, an offer which they accept. Lillie's family is also caught up in the blizzard and decides to camp out for the night, using an inflatable tent made by the Aether Foundation as shelter.
In the morning, once the weather has cleared up, Magearna points their way towards a nearby house. Eager to see her father again, Lillie rushes there ahead of the others. When she reaches the house, she calls out to see if anyone's home. She starts hearing piano music coming from inside, but before she can investigate it, the front door opens to reveal a man whom Lillie immediately recognizes as Mohn. Overjoyed, she hugs him and tells him that she's his daughter, realizing that he last saw her when she was still a baby. The rest of the family reaches the house, and Zoroark happily nuzzles its former Trainer. The family is surprised, however, when Mohn not only fails to recognize his wife, children, and partner, but is also surprised by them knowing his name, causing Lusamine to almost faint from shock. They explain the reason behind their search, and Mohn, understanding they've come a long way to get here, invites them inside for some tea.
Inside the dimly lit house, Lusamine and her children come to understand that Mohn has amnesia. Just as Mohn serves the family some tea and the Pokémon some food, piano music starts sounding from a different room. To the family's shock, Mohn reveals that the music is being played by his daughter, who, by coincidence, is also named Lillie. Mohn takes everyone to see her, but when they enter the piano room, Lillie's family immediately notices that the thing playing the piano is not a human, but a Shiny Nihilego. Gladion almost has Zoroark attack it, but is stopped by Lusamine as the Nihilego floats away and turns invisible. Mohn apologizes, saying that his "daughter" is shy, and leaves to brew some more tea. Left alone with her children, Lusamine explains that she stopped the attack because Mohn views the Nihilego as his daughter, so attacking it wouldn't be a good idea. She goes back to talk with Mohn, while her children start searching the house for clues. Meanwhile, Ash's Pikachu catches a familiar scent and starts leading Ash, Goh, and Chloe towards Mohn's house.
Lillie starts her search from the piano room, with Nihilego reappearing behind her without her noticing. She opens the dirty windows to get some light into the room and starts going through a drawer. Gladion, meanwhile, finds out that mirrors around the house seem to have been removed from where they should be. Inside the drawer, Lillie finds Mohn's old Aether Foundation uniform and a mirror, from which she notices the Nihilego behind her, about to attack her with Venoshock. Just before the Venoshock can go off, Ash's Pikachu jumps in through the open window and uses Iron Tail, forcing the Ultra Beast to dodge and disappear again. Lillie is surprised to see Ash again, as is Gladion as he enters the room. Goh, meanwhile, recognizes Lillie's name and tells Chloe about her while Lillie shows her brother what she's found.
Meanwhile, Lusamine shows Mohn a family portrait of them together, but he fails to see any similarities between himself and the man in the photo, leaving Lusamine heartbroken. She tries to tell Mohn that his "daughter" is actually an Ultra Beast, but before she can finish her sentence, Nihilego appears and tries to use Venoshock on her, only to be narrowly saved by Gladion as Zoroark pins Nihilego down. It tries to disappear, but is stopped by Lillie coming in and thanking it for rescuing her father. Ash, Goh, and Chloe also enter the room, bringing with them the mirror and the uniform that Lillie found in the piano room. Gladion, Lillie, and Lusamine explain to Mohn that he was once caught in an accident involving Ultra Wormholes, which caused him to disappear into another world, where Nihilego found him and brought him back to his own world, except instead of returning to Alola, they wound up at the Crown Tundra, and have been living in this house ever since, as father and daughter.
This conversation makes Nihilego remember how it had witnessed Mohn falling into Ultra Deep Sea, where it had caught him and seen his memories. Barely conscious, Mohn had only been able to say the name of his daughter. Noticing another wormhole, Nihilego had brought Mohn back to his own world and sought shelter in an abandoned house, where it had used its powers to make him see it as Lillie. Gladion theorizes that Nihilego had been happy living with Mohn and had dirtied the windows and removed all the mirrors in the house to prevent Mohn from seeing his own reflection and having his memories return to him. Lillie understands that Nihilego cherishes Mohn, explaining why it had hidden the mirror and Mohn's uniform, as it had feared that Mohn regaining his memories would prevent them from living together.
Gladion presents the mirror to Mohn, who, upon seeing his reflection for the first time in years, instantly remembers everything, including his life with "Lillie", whom he now sees as the Nihilego that "she" was all along. He addresses each of his family members by name, reducing Lusamine to a sobbing mess as her husband finally remembers her name, and the entire family shares a group hug, with Mohn expressing his surprise at seeing Magearna moving again. Nihilego turns away and starts to leave, but Lillie grabs it and asks for it to stay. Mohn agrees, as it's thanks to it that he's been able to reunite with his family, but that doesn't mean they can't continue to live together. Gladion and Lusamine wholeheartedly agree to this as well, and so, with the whole family holding out a Beast Ball together, the "other Lillie" lets itself be caught and officially joins the family. Chloe celebrates the occasion by taking a group photo of the newly reunited and expanded family with her Rotom Phone.