In 1782, in order to oppose the power of the Noron clan, the King Jeongjo of Joseon investigated corruption in various places and appointed Choi Se-geun (Kim Myeong-min) as the detective. He was an outstanding detective in the history of Joseon. A household was killed for exposing corruption. Choi Se-geun cleverly found the real murderer who was color blind, but the murderer was immediately killed by an unknown killer with an iron needle imported from Qing Dynasty, and the case came to a stalemate. After Choi Se-geun recruited Han Seobil (Oh Dal-su), a master of dog theft, as his assistant, he took over the task of investigating the case of the martyred woman. Kim Hyun-young, the niece of Ren Panshu, the leader of the Noron clan, committed suicide for chastity. During her lifetime, she opened a mountain aconite farm, promoted the concept of equality, and was deeply loved by slaves. After Kim Hyun-young's death, the farm was handed over to the beautiful Han Shanghang (Han Ji-min) to manage, and Han Shanghang was also inextricably linked to the iron needles of Qing Dynasty. Ren Panshu, who hates Catholic forces, also has a close relationship with Han Shanghang. The clues of the corruption case and the heroine case are intertwined and point to Han Shanghang at the same time...