A Letter Hiding the Bright Moon

A concubine-born daughter, treated as a pawn by her family, uses her perfect memory and a stationery shop she's quietly built up to establish an intelligence network in the capital. While investigating an unsolved case, she unexpectedly becomes the “wife in name only” of a young lord who seems like a spoiled wastrel but secretly commands a private guard. As they test each other, they uncover a conspiracy and ultimately turn the entire board of the realm upside down.