Forbidden Quest (2006) on Netflix

Plot summary

In Joseon-era Korea, Yun-seo serves as an inspection officer. His brother has been beaten nearly to death, and his elders want to answer the family they blame with a false appeal to the King; Yun-seo refuses to write it. At court, the King gives him a case: a painting by his concubine Jung-bin was mounted, then replaced with a copy, and only a missing scribble on the back gave the switch away. Yun-seo is struck by the forger's skill, and Jung-bin, pleased by his knowledge of art, invites him to speak with her again. He works the case with Gwang-hun, and their search takes them into a shop, where an old man in the basement is copying out an erotic book. Yun-seo reads a few lines, at first shocked, then curious.