The Secret Princess! (Japanese: リーリエと秘密の機巧姫！Lilie and the Secret Mechanical Princess!) is the 116th episode of the Sun & Moon series, and the 1,055th episode of the Pokémon anime. It first aired in Japan on April 7, 2019 and in the United States on August 10, 2019.
Lillie and Gladion are at Aether Paradise, searching for the research files written by their father, Mohn, prior to his disappearance into an Ultra Wormhole years ago. Lusamine regrets not telling her children more about Mohn, but she decides now is the right time. Back at home, the siblings explore Mohn’s room and find a couple of surprises—a Z-Ring, which Lillie holds on to with Kahuna Hala’s blessing, and Magearna, an Artificial Pokémon that Mohn intended as an eventual gift for Lillie. Unfortunately, Magearna is inactive and completely silent—but Lillie, full of gratitude, vows to continue her father’s work to get it moving again!
As Ash deals with Meltan eating random objects throughout Professor Kukui's house, Lillie and her brother Gladion search through his documents for their father's experiment notes. Faba affirms that Mohn's research was transferred to his lab after he was involved in an accident. He then reveals that Lusamine attempted to have the research made confidential, much to the shock of the siblings. Faba bursts into tears over Mohn's death, but Lillie and Gladion are quick to clarify that their father is still alive. As Faba looks for a tissue, he locates an old photo of himself with Wicke, Lusamine, and Mohn. Meanwhile, Professor Burnet and Wicke enter Lusamine's office to report that they have received no new information on Mohn's whereabouts. Burnet admits that the search has only just started, offering Lusamine some comfort. Lusamine admits that since Gladion returned from Poni Island with the revelation that Mohn is still alive, she has felt guilty for not telling her children about their own father. Lusamine then enters Faba's department and calls for her children to come with her, leaving Wicke and Burnet to assist Faba in the search for the research papers.
Lusamine flies her children over to the family's Melemele Island mansion, where Hobbes greets them. As they enter the house, Hobbes reveals a secret chamber with a connected basement hidden behind a bookshelf. Lusamine escorts everyone into the basement and reveals that it is Mohn's research area. She then entrusts the room to Lillie and Gladion until Mohn's return. Lillie notices an open drawer, and inside, she finds a Z-Ring. Hobbes tells Gladion and Lillie that it belonged to their father, and Hala gave it to him upon his arrival to the Alolaregion to research Ultra Wormholes. He further explains that Mohn was a powerful Pokémon Trainer. This leads Lillie to ask where her father's Pokémon are, and Lusamine reveals that it ran away following its overwhelming feeling of loss from Mohn's disappearance. Finally understanding her mother's emotions, Lillie urges her not to blame herself so harshly. Lusamine is touched and admits that Lillie and Gladion kept her focused. She attempts to hug her daughter, only for Lillie to push her away, claiming that she isn't a baby anymore. Gladion than hands Lillie the Z-Ring and suggests she talk to Hala about it. Lillie accepts and Snowy appears to be excited by the prospect of using Z-Moves. She suddenly notices an ornate cabinet and her curiosity leads her to open its twin doors. Inside is a mysterious metallic object, which Hobbes reveals is the Artificial PokémonMagearna. Lusamine reveals Mohn got Magearna in an antique store and was going to give it to Lillie, but it was too rusted to work.
The following day, Lillie shows off her father's Z-Ring to her classmates at the Pokémon School. Professor Kukui informs the class that Lillie will be talking to Hala about the Z-Ring, and she clarifies that she wishes to use it until her father's return. As a result, the whole class and Professor Kukui decide to visit to Hala's Iki Town home. Hala recalls when he gave Mohn the Z-Ring, and admits that Mohn and his Zoroark proved to be tough competitors against his Hariyama. Lillie then informs Hala that she recently found her father's Z-Ring and asks to use it to help herself get stronger. Sensing the emotion in Lillie's request, Hala asks her to perform Subzero Slammer in front on him. Everyone is shocked by the request, though Lillie accepts the challenge.
On the grand trial battlefield, Lillie straps her father's Z-Ring to her wrist and places her Icium Z into it. She recites the Z-Move chant and performs the correct pose. The Z-Power is passed onto Snowy, but at the last moment, it disperses. Ash urges Lillie to keep training, and Kiawe reminds her that the Alola League is just around the corner. Sophocles and Mallow are also impressed by her efforts, with Sophocles even showing interest in wanting to perform a Z-Move himself one day. Snowy walks over to Lillie, feeling disappointed, but Lillie reassures her that she will become stronger and master the Z-Move. Hala is impressed nonetheless and grants Lillie his permission to wield her father's Z-Ring.
Lillie returns home and begins speaking to Magearna. She informs Magearna of the day's events before asking it to be her friend. As Lillie confesses her desire to fulfill her father's wishes, Snowy notices some light flicker in Magearna's eyes and heart, although this goes unnoticed by Lillie. As Lillie looks closer, she notices a book tucked behind Magearna. She flicks through a few pages of her father's journal, noticing a drawing of Magearna holding her baby self, filling her eyes with tears as she moves on to find diagrams showing the inner workings of Magearna. Suddenly the lights turn on, and Lillie turns around to see Lusamine and Gladion standing at the doorway. Lillie reveals that she received Hala's permission to use the Z-Ring and she vows to train hard. Lusamine hones in on the book in Lillie's hands, and Lillie explains that it appears to Mohn's diary on Magearna. Lillie then asks if she can take care of Magearna in the hopes of getting it to move one day, and her mother agrees to it. Overnight, Lillie reads about the moment her father located Magearna in his journal and soon drifts off to sleep.