Kkok Du was once a mortal living in ancient Korea, but the gods transformed him into an immortal grim reaper whose duty is to punish humans every 99 years for their weaknesses and imperfections. After centuries of carrying out his ruthless duties, Kkok Du has grown to hate humanity and has become cruel and merciless. In his latest visit to the mortal realm, Kkok Du takes possession of the body of a talented young doctor named Do Jin Woo. However, his mission to reap souls is complicated by his need to carry out Jin Woo's medical duties, which brings him into contact with Han Gye Jeol, a doctor from a low-ranked medical school in South Korea. Despite her unassuming background, Kkok Du discovers that Gye Jeol has mysterious powers and seems to have a strange hold over him. As Kkok Du tries to unravel the mystery of their connection, he begins questioning his purpose and whether his mission to punish humans is genuinely just. As Kkok Du struggles with his conflicting emotions, he must also navigate the dangerous forces that seek to undermine his mission and threaten his existence. Will he complete his duty, or will his growing attachment to Gye Jeol derail his mission and put them both in danger?