Early in his reign, King Se Jong accidentally causes the death of his in-laws and their slaves in an attempt to save them from his brutal father, King Tae Jong. Two young slaves, Ddol Bok and his friend Dam, survive but neither knows the other has. Ddol Bok blames the king and vows to kill him to avenge the deaths of everyone he loved. He spends several years becoming a great warrior. Dam, who has a photographic memory, becomes a court lady called So Yi, but she feels so guilty about her role in the other slaves' deaths that she is mute.
Ddol Bok returns to the palace under the name of Kang Chae Yoon in order to kill the king. However, he becomes embroiled in the mystery surrounding the deaths of several Hall of Worthies scholars. The deaths were caused by a secret society called "Hidden Root," which was created long ago by followers of Jeong Do Jeon, with the goal of giving more power to ministers and less to the king. King Tae Jong killed Jeong Do Jeon and all of his family except his nephew, Jeong Gi Joon, who is now the leader of Hidden Root and lives in disguise not far from King Se Jong.