A new Ultra Beast has appeared from the Ultra Wormholes and has started attacking trainers. When attacking them, it ends up stealing their Z-Crystals. The Ultra Guardians are tasked with stopping this Ultra Beast, Pheromosa. However, Pheromosa also appears and steals Team Rocket's Z-Crystals, but while trying to stop it Meowth falls in love with Pheromosa. Will everyone be able to stop it before it's too late?

Since their trip to Poni Island made them realise that their father, Mohn, is still alive, Lillie and Gladion return to the Aether Paradise and investigate their father's old office. When there, Lillie discovers her father's old Z-Ring and the Mythical Pokémon, Magearna. Lillie approaches Hala and asks if it's alright for her to use her father's Z-Ring, and so they have a battle to see if Lillie can master the Z-Moves. Will they find more information about their father's location?