Gang-du is a half-witted man, working at his father's snack bar near the Han River in Seoul. The biggest joy of his life is his only child, daughter Hyun-seo, a brave and smart middle school girl. One day, outside their little shop, crowds are staring at an unidentifiable object hanging from a nearby bridge. In an instant, the object reveals itself as a terrifying creature turning the riverbank into a gruesome sea of blood. Unfortunately, the creature snatches the helpless Hyun-seo with it down underwater, and as the government does nothing to save her, Gang-du and his family are forced to find their inner hero and somehow bring her back.