Choi Si-woo is a young man with a huge complex about his weight, because of this he has gone under serious training routines and diets to finally be able to gather the courage in himself to declare his feelings towards his crush, but when he does and his crush rejects him, he intensifies his training and decides to run for hours in the night, however, a mysterious old man in a cloak suddenly appears in front of him and teleports Si-woo to the ruins of an old temple, there he's told that he is the last human with goblin blood inside and because of this he is the only person capable of claiming the title and the powers of the goblin king in the ceremony carried every 666 years to elect a new ruler called "The night of the goblin".