Flu

Smugglers Ju Byung-woo and Ju Byung-ki discover that illegal immigrants died in a shipping container from an unknown illness. They capture the sole survivor, Monssai, and bring a cellphone video of the bodies to their boss. Byung-woo grows sick, and Monssai manages to escape.

The brothers take the survivor to a clinic where Byung-ki is questioned about the container. The contagion spreads as the virus mutates, and hospital staff begin to fall ill. Byung-woo dies, and Byung-ki refuses to answer questions about the container.

Dr. Kim In-hae at the Contagion Center reviews the cellphone video and theorizes that conditions in the shipping container allowed the virus to mutate. She is reprimanded for losing important data after her car fell into a mine shaft the day before.

Oh Ji-goo and Bae Kyung-ub retrieve In-hae's bag from a mine shaft and deliver it to In-hae's daughter Mi-reu. The personal connection spurs In-hae to monitor the outbreak more closely and to share her early suspicions about the virus.

Byung-woo's condition worsens and he is taken to an emergency room where he is isolated with an unknown flu. Byung-ki coerces hospital staff and refuses to answer questions about the shipping container, driving the tension within the hospital.

The next day, many people show pronounced symptoms as the outbreak spreads. Hospital staff and the KCDC incinerate the shipping container, but rats feeding on the corpses escape into the city, spreading the contagion further.

Monssai rescues Mi-reu from being hit by a car, aware he might be spreading the virus. Mi-reu calls Ji-goo to help search for the sick man, but they have no success. Rumors of the outbreak spread and people begin to panic.

Administrators and politicians are pressured to quarantine as hospitals and communications systems become overwhelmed. The Prime Minister makes a public announcement that worsens the panic in Bundang.

In-hae and Ji-goo search for Mi-reu and find her in a looted supermarket as riot police try to contain the outbreak. They escape before steel shutters come down, but Mi-reu's symptoms prevent passing to Seoul.

Night falls as the Bundang quarantine is reinforced by the army, US forces, and KCDC, and the population is moved to a camp outside the Tancheon Sports Complex with an infected quarantine zone beneath the stadium. In-hae secretly begins a transfusion to Mi-reu after an official decision blocks direct antibody injections.

On the second day, Monssai is found in the isolation zone. Gook-hwan leads an infected mob toward the highway, triggering a gun battle between the mob and soldiers. Byung-ki kills Monssai in a suicidal attack to avenge his brother.

Mi-reu begins to recover and is sheltered by Ji-goo and In-hae as they move toward Seoul. The president orders containment to end and cancels an air strike, changing the crisis's course. Mi-reu is sent to Seoul to help create a vaccine, while relief teams are dispatched to Bundang.