In the bustling city of Seoul, South Korea, the lazy and dim-witted Gang-du works at his father's small snack bar on the banks of the Han River. One day, their peaceful life is shattered when a monstrous creature emerges from the river, turning a relaxed afternoon into a scene of terror. The creature rampages through the riverside park, devouring people and causing panic. As the city descends into chaos, Gang-du's young daughter, Hyun-seo, is snatched by the creature and presumed dead by authorities and the public alike.

Amidst the subsequent government lockdown and a public gripped by fear, Gang-du receives a call from Hyun-seo, who is alive but trapped within the sewer system where the monster has made its lair. Determined to rescue her, Gang-du joins forces with his father Hee-bong, his archery champion sister Nam-joo, and his alcoholic brother Nam-il. The family members, each dealing with their personal failures, must overcome their inadequacies and government obstacles in a desperate and dangerous quest to save Hyun-seo. Critique of government incompetence and environmental recklessness is woven throughout the narrative as the family's resilience and love are tested in the face of the terrifying and seemingly unstoppable creature.