Bo Sal, a monk at the Hwaguksa Buddhist temple and head of the Hwaguksa hitman group, picked up Bi Gwang as a child and raised him as a hitman.

When Bi Gwang grew tired of the killing and decided to leave the group to start living a normal life, Bo Sal and the other members of the Hwaguksa group betrayed and killed him.

After his death, Bi Gwang's soul was transferred into the body of a police officer named Kim Gyeongsu and he vowed to take revenge on the Hwaguksa group for their betrayal.