The film revolves around Gang-du, a dim-witted man working at his father´s tiny snack bar on the banks of the Han River. Following the dumping of gallons of toxic waste into the river, a giant mutated squid-like Monster suddenly emerges from the depths of the Han River and begins attacking people.
Gang-du´s daughter Hyun-seo is snatched up and carried off by the creature. He and his family are then taken into custody by the military, who fear a virus is spread by the creature. In detention, Gang-du receives a phone call from Hyun-seo, who is still alive, but trapped in a sewer.
With only his family to assist him, he sets off to rescue his daughter..