There are some heroes whose names you do not know, whose faces you have not seen... They operate secretly, and their actions can make a difference in the outcomes of wars. Biheun is the story of such a man. To follow his story, one must search in the shadows, for that is the only place you could find him...

Kang In-Young, betrayed by his friend and his lover, is framed for murder and is thrown into prison. There he meets a fellow prisoner, an old man who used to be a petty thief. Turns out he stole a secret martial art from JeonJin sect, and to escape their pursuit, he intentionally broke the law to enter the prison and spent 30 years honing the secret martial skills. Tragically the old man catches a plague-like disease and is going to die… but after observing Kang, he decides to make him his heir and gives him the secret martial arts. 10 years after the old man's death he fully masters the secret art…