The main story of YiHuan revolves around the player character — the Appraiser. The Appraiser is a Cosmos-attribute Esper with special appraisal abilities, recruited by Hotori, the owner of the Eibon Antique Shop. The shop appears to be an ordinary antique store but is actually an outsourced agency of the Bureau of Anomaly Control, responsible for handling various anomalous commissions in Hethereau.
At the antique shop, the Appraiser works alongside Espers of diverse personalities: the taciturn swordsman Zero, the passionate vampire-type Esper Fadia, the lazy Construct controller Lacrimosa, and the butler Adler with his companion Sunyas. Each Esper has their own story and secrets that are gradually revealed as the plot progresses.
The central conflict of the main story revolves around the conspiracy of The Circle organization. This mysterious group appears to have mastered methods of creating and controlling anomalies, though their true purpose remains unknown. While handling anomalous commissions, the Appraiser gradually discovers that these seemingly independent incidents hide a larger conspiracy. As the investigation deepens, the Appraiser's appraisal abilities continue to evolve, revealing deeper secrets about the origin of anomalies and the awakening of Espers.