The Han River runs through the heart of Seoul and when the US military dumps toxic chemicals in the river, a monstrous mutant creature is conceived in its waters. Gang-doo, who operates a snack bar on the riverbank, witnesses his only daughter, Hyun-seo, taken away by the creature right before his eyes. The sudden and shocking appearance of the mutant creature renders the government and state powerless to act. The once-ordinary citizen, Gang-doo, and his family, sneak past the military cordon and are thrust into a battle with the monster in a desperate attempt to rescue Hyun-seo.