In a small town, a young boy named Choi Si-woo grapples with his struggles, feeling burdened by his weight. To escape his worries, he runs around his house late into the night, seeking solace in the stillness of darkness. One evening, as he explores a nearby walking trail, he unexpectedly encounters an elderly man carrying a straw backpack. In a swift motion, the old man covers Si-woo with the backpack, transporting him to a vast, ancient traditional Korean house (hanok). Inside the courtyard, countless dark figures surround Si-woo, leaving him bewildered and anxious. The old man reveals the truth – every 666 years, a ceremony takes place to select the king of Yokai, and every Yokai possesses an entry ticket. Remarkably, Choi Si-woo is not just a regular human; he is a hybrid of a human and an extinct goblin, making him the sole possessor of the coveted entry ticket. As the last surviving link to the goblin folk, the adventure of the half-monster boy, Choi Si-woo, begins in this mysterious world of Yokai.