The original Korean title's literal translation is Secret Investigation Records, and the series is also known as Joseon X-Files: Secret Book (조선X파일: 기찰비록; JoseonXpail: gichalbirok).
In this Joseon-era alternate history, Kim Hyung-do (Kim Ji-hoon) is an inspector with a strong focus on investigating into administrative misdeeds. This brings him into direct contact with a secret organization within the government. Dealing with strange, paranormal and possibly alien phenomena, the duty of this organization is to keep everything under control, leaving only secret records. Occurring during the reign of King Gwanghae, the episodes make various allusions to the political turmoils of the period. These individual stories are:
E01–02: The Secret Light (Parts 1 & 2)
E03: The Curse of Shilla's Gold
E04: Behind Those Crimson Eyes
E05: The Sealed Tale
E06: Ghosts at Yi Du's House
E07: The Four Dimensions Village
E08: The Immortal Prophet
E09: Attack of the Changgwi
E10: Return to Dragon Palace
E11-E12: The Rebirth of the Dubak God (Parts 1 & 2)
In these various contexts, investigator Kim Hyung-do avoids superstition and searches for realistic explanations while investigator Heo Yoon-yi (Im Jung-eun) is more open to supernatural possibilities, including secrets of her own. The finale doesn't explain everything. Kim Hyung-do's last reply to Heo Yoon-yi is only "I know who you are".
