Death has her second in command, an entity who has been in charge of taking the souls when it's the right time. 

Moonbyul is the general shinigami  for the last 2500 years.

She never interacted with a living creature because her soldiers are the ones who get down to Earth.

Until Death's right arm sees the light of a human from up there and she gets curious about this person .

What happens when the shinigami Moonbyul walk on Earth and is seen by the human Yongsun?

Two worlds that should not have interaction collide