Gang-du, played by Song Gang-ho, works in a kiosk alongside his dad in the Hangang riverside park. One day, he discovers something large and animal-like dangling from the underside of a bridge, with people crowded about, staring at it. Suddenly, the unidentified monster jumps down onto the park pathway and starts crushing, trampling and eating people. In an instant, it's running around in a frenzy.
Gang-du begins to run away with his daughter, Hyun-seo, played by Go Ah-sung. Along the way, however, he accidently loses hold of her hand. The monster seizes the moment and snatches up Hyun-seo into its maw, taking her away as it dives back into the river. Police, the army, relevant authorities and even the U.S. Army decide to close the river, wary of a certain virus. The whole city becomes paralyzed. Gang-du receives a phone call from Hyun-seo while at the hospital being tested for any possible virus, and learns that she is still alive. Government authorities, however, do not trust him and discount his words. They think he suffers from a certain type of delusion, a possible symptom of the suspected virus. The family has no choice but to rely on each other. His sister Nam-ju, an archer, played by Bae Doona, and his younger brother and a former student activist, played by Park Hae-il, decide to escape from the containment zone, filled with infected people, and leave to seek out and save Hyun-seo.