A fantasy medical drama about a high-ranking grim reaper named Kkok Du, who is the only being to exist in the underworld. Every 99 years, the cold and cruel Kkok Du must take a break from his job in the underworld and spend 49 days in the mortal realm—and this time, he winds up inhabiting the body of the human Do Jin Woo. While living as Do Jin Woo, he meets Han Gye Jeol, a female doctor who has the mysterious ability to order Kkok Du around.
Kkok Du, once a mortal man in ancient Korea, is now an immortal reaper, revisiting the mortal world every 99 years to punish human weaknesses. His millennia-long existence has hardened him, cultivating a ruthless disdain for humans. During his latest visit, Kkok Du inhabits the body of a gifted young doctor named Do Jin Woo.
However, his mission becomes intricate when he must perform Jin Woo's medical duties. In an unexpected twist, Kkok Du encounters Han Gye Jeol, a doctor from a lesser-known school, yet harboring enigmatic powers that captivate him. Are their destinies intertwined from a past life, and why does she wield such influence over him?
As Kkok Du grapples with his task, he finds himself increasingly drawn to Gye Jeol, facing a daunting choice: fulfill his duty or risk everything for love.