It’s the 22nd year of Jung Jong. The plague has taken over Joseon, 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, Jung Jong brings back his most trusted general Yoon Gyum from retirement. Joined by his daughter Myung, his right-hand man Sung Han, and royal court officer Heo Yoon Gyum sets out to find the mysterious creature. Is the creature, Monstrum real? Or a figment of the fearful mass’ imagination? Will the people of Joseon ever live in peace again?