A Poipole flying low over the ocean spots a Chinchou and hits the unsuspecting Pokémon with a poison attack to get its attention. Chinchou responds with Thunder Wave, only energizing Poipole and scaring the wild Chinchou away. Poipole reminisces about Pikachu using Thunderbolt against a Buzzwole a while back. Its impulse leads it to seek out Ash and Pikachu, it soon locates them on their way to the Pokémon School the next morning.
Class has begun and Sophocles is presenting a series of photographs taken by Molayne, Sophocles's cousin and the observatory director, using the Hokulani Observatory on Ula'ula Island. Professor Kukui points out his friend Molayne in a picture and notes he is a talented Trainer. In the midst of the presentation, Pikachu receives a face full of blue liquid and notices Poipole looking into the classroom. Pikachu abandons the class presentation to do some investigation, not that anyone notices. Sophocles talks about his desire to discover new Pokemon from outer space and concludes his presentation. Lunchtime arrives, with Ash and friends sitting with their desks arranged in a circle. Ash brings out Lycanroc, Rowlet, and Torracat for a hand-made lunch, courtesy of Professor Burnet, and suddenly realises Pikachu's absence. Everyone decides to search for Pikachu around the school grounds. Rowlet's eagerness leads Ash to search outside the school, while everyone else keeps up the search in the school. Meanwhile, Pikachu has run outside of the school grounds, chasing Poipole into a nearby forest. The pair stop in a cleared path, Poipole goes on to nuzzle Pikachu much to the yellow Pokemon's enjoyment. Poipole then fetches an apple from a nearby tree, which Pikachu splits into two with an Iron Tail.
In the middle of the search, Rowlet becomes distracted and swoops into Team Rocket's honey donut food van for a snack. Jessie calls upon her Mimikyu to use Wood Hammer, the attack hits and sends Rowlet flying off into the distance. Rowlet breaks its uncontrolled spin mid-air and notices Pikachu and Poipole in the forest clearing. The Flying-type heads directly for Poipole, a perceived enemy, though its attack is disrupted by Poipole's defensive Toxic attack. Pikachu runs over to the now-poisoned Rowlet and begins to spark with electricity. Pikachu stops its sparking after hearing Poipole's teary plea. Poipole instantly grabs Rowlet and Pikachu leads the troupe back to the school. Team Rocket notices the mysterious purple pin-headed Pokemon, Pikachu, and Rowlet, all walking past from their food van. Meowth proves unable to translate Poipole's speech, admitting he doesn't recognize it. James, with Mareanie on his shoulder, checks his logbook for any matches, to no avail. Team Rocket decides to follow after suspecting the Pokemon is rare, or maybe an Ultra Beast.
Soon enough, Team Rocket catches up to Poipole, who is taking a break out of exhaustion from carrying Rowlet on its back. The trio launch into their classic motto, only to realize they committed to their ritual in front of Pokemon, not people. Poipole's laughter at Team Rocket only infuriates the trio. Meowth produces a net launcher, though Ash's Lycanroc and Torracat arrive on the scene destroying the net in the process. The new arrivals take Team Rocket by surprise, though when Meowth sees the newly-evolved Torracat for the first time, he is seemingly excited about it. Ash and Rotom Pokédex soon walk onto the scene, instantly noticing Poipole. Rotom takes a frenzy of photos, revealing it has no data on the Pokémon. James calls on Mareanie for assistance, though the Pokemon instead grips onto his head and poisons him. Out of frustration, Jessie calls out Mimikyu. Pikachu dodges Mimikyu's Shadow Claw and retaliates with Iron Tail, but the attack fails to do any damage, merely activating Mimikyu's Disguise ability. Mareanie's Spike Cannon collides with Lycanroc's Rock Throw. Jessie attempts a Shadow Ball though the attack misses. Before Pikachu's Thunderbolt hits Team Rocket, Bewear intervenes and scoops the trio up into its arms and out of harm's way.
In the evening, Ash returns to the Ultra Guardians base, underneath the Pokémon School. Lillie heals Rowlet's poisoning. Poipole continues to nudge Pikachu and removes Ash's Z-Ring from his wrist. Suddenly, Lusamine, Wicke, and Professor Burnet appear via a projection display. Lusamine announces that thanks to Rotom's footage, they have identified the mysterious purple Pokemon as an Ultra Beast called Poipole, the Poison Pin Pokemon. Rotom, with Lusamine's word, goes on to officially register Poipole in its system. Lusamine also grants Ash custody of Poipole to allow for daily observation until she and her team can figure out which Ultra Wormhole it appeared from. Lusamine's Clefable brings out the Beast Balls for Ash to capture Poipole. Despite Ash's welcoming speech to encourage Poipole to join his team, the Pokemon avoids all of the Beast Balls. Lillie suggests that Ash demonstrates how Poké Balls operate. Ash uses Rowlet to demonstrate, so he sends out and returns the Pokemon several times. Unexpectedly, Poipole flies up to Ash and touches the Beast Ball's central button, causing it to enter the device. The Beast Ball rocks on the ground several times before settling, to everyone's amazement; this officiates Ash's capture of Poipole.
That night, Ash and his Pokemon are excited with Poipole, the latest edition to the team. Rowlet, with Pikachu on its back, chases Poipole through Professor Kukui's home before eventually stopping out of exhaustion. Poipole goes on to spray several lines of blue and red fluid from its head onto one of the log walls, producing a portrait of Pikachu. The captured likeness impresses Pikachu and the others. Professor Kukui, in talking to Professor Burnet, suggests Poipole may prove to be a lot of effort because of its energetic nature and prankster antics.