Keisuke gets dragged into his sister's secret life and immediately becomes the skeptic audience surrogate, which means every youkai reveal lands twice as hard because he doesn't want to believe any of it. The show trades the mascot-collection formula for actual mystery work, treating supernatural cases like real detective procedurals where the weird stuff matters because someone has to solve it. A kid who thinks this is all nonsense watching his sister take it completely seriously is the engine that keeps this running.

Keisuke's a skeptic through and through. At eleven years old, he doesn't buy into ghost stories, alien sightings, or any of that youkai nonsense. So when his older sister Natsume keeps coming home late, he gets suspicious enough to follow her. What he finds instead of a reasonable explanation is a youkai detective agency, and suddenly Natsume's secret is out. Now Keisuke's pulled into a world he never believed existed. Youkai are real, they cause real problems, and his sister has been fighting evil ones and investigating their crimes with her friends Touma and Akinori while he was busy dismissing them as folklore. With her secret exposed, Natsume lets Keisuke tag along. Set after the fourth film, with the Amano siblings as children of the original series' Keita and Fumika, the cases keep coming, the youkai keep causing chaos, and his skepticism gets tested daily as the group works to keep the city at peace.