It’s the 16th Century. The plague has taken over and fear runs rampant in the streets. When rumors of a vicious beast roaming Mount Inwangsan—called “Monstrum” by terrified masses—begin to spread, fear turns into panic. In order to quell the rising panic, King Jungjong brings his most trusted general, Yoon-gyeom, back from retirement. Joined by his daughter Myeong, his right-hand man Seong-han, and royal court officer Heo, Yoon-gyeom sets out to find the mysterious creature. Is the creature, Monstrum, real? Or a figment of mass imagination?