Gyeongseong Creature is set in 1940s, during the Japanese occupation of Korea. The show draws inspiration from Unit 731, the infamous unit that did experiments on their captive population. In this historical fiction drama, the show depicts the creation of a monster by the Japanese and how Jang Taesang (Park Seojoon), owner of a famous pawnshop in Gyeonseong, and sleuth Yoon Chaeok (Han Sohee) become intertwined with the monster and each other.