Koumuin de Antai no Hazu ga, Jikobukken Tantou ni Idoushimasu

Aire, a lonely girl searching for her mother's keepsake, enters a masked auction house with an anonymous invitation in search of clues. Paintings, jewels, antiques… As various items are auctioned off, Aire, with her unique aesthetic eye, detects that forgeries have been mixed in among the merchandise. This leads to her talent being recognized by Bartraut, the young head of the Marquis family that runs the auction. She joins the auction group, "The House", as a member of the team, assisting them alongside the gentle fifth prince, Felix, and the cheerful second son of a baron, Joshua!

Initially, Aire dislikes Bartraut's condescending attitude. However, she gradually opens her heart to him as he sees through her inability to rely on others and, though clumsy, extends a helping hand. However, at the same time, she finds Bartrud's obsession with authenticity to an almost abnormal degree to be unsettling. It seems the reason for this is deeply connected to why he's hosting the auction...?

(Source: Kadokawa, translated)