Set in the fictional county of Jinyang, 'Monstrous' follows archaeologists, detectives, and residents as they become trapped in a terrifying phenomenon caused by the discovery of a mysterious Buddhist statue possessed by an evil spirit. Jung Ki-hoon, a brilliant archaeologist who investigates bizarre supernatural phenomena, is drawn to the county due to strange occurrences surrounding the unearthed artifact. Alongside his estranged wife Lee Soo-jin, a cryptogram analyst haunted by a personal tragedy, they must navigate a town where citizens are driven mad by their deepest inner fears and violent delusions. As the dark entity spreads its influence, chaos erupts, cutting off the town from the outside world. Local authorities, including a dedicated police officer and the county's governor, struggle to contain the escalating madness while trying to save survivors. The series explores human psychology, guilt, and resilience as the characters are forced to confront both external supernatural horrors and their own internal demons in a desperate fight for survival.