In Seoul, South Korea, securities manager Seok-woo, recently divorced, struggles to care for his daughter, Su-an. On Su-an's birthday, he agrees to fulfill her wish to visit her mother in Busan, traveling via the KTX high-speed train. Meanwhile, a mysterious virus begins spreading nationwide, turning the infected into aggressive zombies who transmit the infection through bites. Initially misidentified by the government as a protest or riot, efforts to contain the situation inadvertently lead to a surge in infections. Onboard the train, a woman bitten by a zombie seeks refuge but soon transforms, attacking crew members and passengers. Chaos ensues as Seok-woo, Su-an, and other survivors retreat to a rear car, narrowly avoiding the infected. In the process, they almost leave behind a pregnant woman, Seong-kyeong, and her husband, Sang-hwa. The couple manages to enter the safe area before Seong-kyeong uses a newspaper to obscure the glass door, preventing further attacks from the zombies outside.