In Seoul, an American army doctor forces his assistant to dump bottle after bottle of formaldehyde into the drain. The drain runs into the Han River, the major river running through the middle of Seoul. His only reason is that the bottles are covered with a layer of dust. Six years later, a food cart operator, Gang-du falls asleep every time his dad, Hie-bong, is away from the stand for more than a few minutes. When his sister Hyun-Seo returns from school, a monster appears in the river and starts to attack the people lounging at its banks. Gang-du and Hyun-Seo are chased by the beast. Hie-bong tries valiantly to save his daughter, but the beast soon captures her and drags her away. They receive a call from the daughter's cell phone and try to rescue her from the large monster.