Using the note given to her by Diana, Liko leads Friede and Roy to Tepen's shop. Seeing a Lampent on the signpost of one of the shops, the group knows that's Tepen's antique store. Upon arriving, Liko tries to tell Tepen why they've come, but Tepen tries to sell them off with products such as a special sleep powder and a "fossilized" Rotom Phone from 20,000 years ago. Tepen becomes shocked when Liko introduces herself as Diana's granddaughter, pointing out that they went on a few adventures back then. After Tepen tells a story of how he and Diana got stuck in an Ariados' web for a whole week, Friede starts to feel suspicious about his stand, but Liko reassures him that her grandmother vouched for Tepen. Roy then shows Tepen the Black Rayquaza's Ancient Poké Ball. Tepen becomes shocked before acting like he's never seen it before, having Roy hand him his whole bag for analysis. He fiddles around with the bag before giving it back to Roy, claiming he doesn't know about the Ancient Poké Ball, so the group decides to leave. As Tepen waves to them, Hatenna senses bad intentions from him and tries to warn the group, but Fuecoco and Roy are too hungry to pay it any mind, so they decide to go somewhere to eat.
While eating at a restaurant, Sprigatito notices something moving in Roy's bag, and a Foongus suddenly pops out and jumps down into the tree below. Roy then panics when he finds out that his Ancient Poké Ball is gone, prompting the group to head back to Tepen's shop to look for it. Tepen, who is revealed to have stolen the Ancient Poké Ball when he swapped it for Foongus, shows it to all his Pokémon before declaring that it's closing time. The group arrives at Tepen's shop, only to find that he's closed up and taken everything with him. Friede believes that it was all part of his plan to take the Ancient Poké Ball without the group knowing, pointing out Hatenna's behavior from before as a warning about his intentions. As they go to catch Tepen, Liko ponders why he would take Roy's Ancient Poké Ball before thinking it might be the Explorers' doing. Meanwhile, Tepen contacts someone about the Ancient Poké Ball.
The group decides to split up to find Tepen, Friede taking Charizard to search by air while Liko and Roy search on the ground. When Tepen spots Friede in the air, he gives the bag with Roy's Poké Ball to Lampent and his other Pokémon. He sends them off as Friede comes down and confronts him. Tepen doesn't come clean, but does let Friede inspect his goods. However, Tepen tips the cart and dumps all his goods onto Friede before making his getaway through a produce market. Charizard goes after him, only to get Friede in trouble with the merchants when it flies through and damages the stands. Friede contacts Liko and points her to Tepen's Lampent having run off with the Ancient Poké Ball before sending Captain Pikachu to help her and Roy. They spot Lampent, joined by Tepen's other Pokémon. Liko sees the bag that Lampent is carrying, thinking the Ancient Poké Ball must be in there. They chase the Pokémon to a museum, where they may be hard to find with all the artifacts that they look like. Liko and Roy eventually find Lampent and the others acting like a single statue, prompting them to run and jump out a nearby window.
When Friede fixes all the produce stands he damaged, he goes off to meet up with the kids, who decide to split up at a park. Roy finds and confronts Tepen's Pokémon. Lampent leaves Bronzong and Honedge to face Roy and Fuecoco. The Fire Croc Pokémon dodges Honedge's attacks and retaliates with Ember. Bronzong then covers Fuecoco with its body and has Honedge hit the Bronze Bell Pokémon so its ringing stuns it, which buys them time to escape and meet up with Tepen. Liko finds Lampent and Bronzor and has Sprigatito use Leafage, only to have it repeatedly blocked Bronzor's Iron Defense. Spotting a shovel, Hatenna uses Confusion to lift it and cut the bag off Lampent. However, before Sprigatito can grab it, Tepen appears and has Lampent use Thief to get it back.
Just as Roy arrives, Liko questions why Tepen is doing this, and Tepen says that someone else came by his stand claiming they'd pay him for the Ancient Poké Ball. Convinced that it was the Explorers after all, Liko and Roy decide to take the Ancient Poké Ball back from Tepen themselves. After Tepen's Pokémon avoid Fuecoco's Flamethrower, Liko has Sprigatito charge at Lampent using Quick Attack to retrieve the bag, but the Lamp Pokémon uses Ally Switch to have Sprigatito slam into Bronzor instead. Fuecoco uses Flamethrower again, but it is doused out by Bronzong's Rain Dance, so Roy has Wattrel use a Spark on Lampent that allows Sprigatito to retrieve it. However, the Grass Cat Pokémon is blown back by Honedge's Metal Sound, knocking the Ancient Poké Ball out of its possession and allowing Bronzong to trap the ball within its body. Things get worse for Liko and Roy when the Foongus from before shows up and puts all of their Pokémon to sleep with the special sleep powder, revealing that it was his Pokémon too. Just when it looks like Tepen has won, Cap shows up, knocks out all of Tepen's Pokémon with a single Thunder Punch, and gets the bag from them. Charizard and Friede arrive to block Tepen's exits, finally making him surrender.
Diana shows up to confront Tepen about the ancient text she was deciphering; it was all fake. Liko stops her grandmother before she can have her Arcanine use Flamethrower on Tepen as payback. Diana then reveals that the adventures they had together were true after all. Tepen claims that thievery is way easier than adventuring, but Diana reveals the truth on Bronzor's crest, the truth that Tepen longs for those adventuring days again. Tepen still thinks he's not cut out for it, but Diana tells him what she told her granddaughter, that the first step is always the scariest part, which doesn't go unnoticed by Liko. Looking at each of his Pokémon, Tepen starts to feel shame for having made them do such bad things, tearfully apologizing to them. Liko becomes amazed by her grandmother, saying that she loves her in her head. At sunset, Tepen decides to make a fresh start with his Pokémon. He also owes someone else an apology, the Poké Ball craftsman who wanted the Ancient Poké Ball. Tepen gives Liko the address to the place where he was supposed to take the Poké Ball, so she decides to head there next.