Warning: This synopsis reveals the plot in full, including the ending.
Korea, the year 2000. An american scientist works in a laboratory with his Korean assistant. They dispose of dangerous chemical waste into the Han River. An amphibious creature is sighted in the river years later.
Korea, 2006. Gang-du, a fast food shack owner is delivering food with his daughter Hyun-seo, when a gigantic amphibious creature emerges from the river attacking and eating people. When trying to escape, the monster grabs his daughter and disappears in the water. The gobernment issues a communicate explaining that the monster is also carrying a deadly unknown virus and that came in contact everyone must be tested. While quarantined at the hospital, Gang-du receives a call from his daughter. She is alive somewhere in the sewers where the creature built a nest. Against all odds, he is able to track down the location of the call and rescue his daughter, who has also helped See-joo, a small boy escape. In the process of escaping, Gang-du’s daughter dies trying to save the boy.
After the creature has been killed, Gang-du adopts See-joo.