The Qing are rumored to dealing in human trafficking of girls from Joseon. Yeo Ji is recruited as an undercover investigator on the case. Prince Yeoning is unhappy at Yeo Ji being constantly in danger. Meanwhile Park Moon Soo, as new member of Saheonbu, is trying to endure all the hard training. The palace officials show that they don't acknowledge the Crown Prince by not showing up at the Prince's first morning lecture.

Jipyeong Oh is murdered and Prince Yeoning is witnessed to be the murderer and it arrested by Saheonbu. He is investigated by Saheonbu but is not willing to speak the truth. Meanwhile, behind the murder cases are what is known as master-murderers and the murderer of Jipyeong Oh turns out to be a little girl wearing a tattoo. Moon Soo and Yeo Ji investigate what's behind the murder cases.