A chemical leak at a biotech plant causes the start of a zombie apocalypse that rapidly spreads across South Korea.Fund manager Seo Seok-woo is a cynical workaholic and divorced father. His daughter Su-an wants to spend her birthday with her mother in Busan. He tries to soothe her with the gift of a Wii forgetting that she already has one. Overcome with guilt after watching a video of Su-an suffering from stage fright while singing Aloha Oe at a recital he couldn't attend, he decides to take her to Busan via an early KTX 101 train from Seoul Station. Other passengers include working-class Yoon Sang-hwa and his pregnant wife Seong-kyeong; selfish COO Yonsuk
a high school baseball team that includes baseball player Min Yong-guk and cheerleader Kim Jin-hee; train attendant Ki-chul; elderly sisters In-gil and Jong-gil; and a homeless stowaway.
As the train departs, signs of chaos from the infection reach the station, and an infected woman runs inside, unnoticed, just before the doors close. She soon turns into a zombie, attacks an attendant, and many crew and passengers are quickly turned into zombies. The remaining passengers quickly secure themselves in the front and rear cars, observing that the infected zombies only attack if they can see them or hear them, but they can't operate the train's doors. They learn from news reports of the rapidly-spreading epidemic across the country, and Seok-woo discovers the biotech plant was connected to his business. He uses his connections to try to secure safe travels for Su-an and himself.The train stops at Daejoon station supposedly secured by the South Korean Army.
The passengers disembark into a vacant station, but soon find the soldiers have become zombies, who quickly charge the group. Several passengers are attacked and killed as they race back to the train, while most of the surviving ones safely board the front cars. Su-an, Seong-kyeong, In-gil, and the homeless man find themselves between two cars of zombies and quickly take shelter in the restrooms. Seok-woo, Yong-guk, Sang-hwa and two of Yong-guk's three surviving classmates, having stayed back to provide cover, race to board the moving train, though Yong-guk's friends are killed and infected during the escape, and end up in the back cars. The captain drives the train to Busan after learning it has been established as a quarantine zone.
Learning of their trapped loved ones, Seok-woo's group uses the darkness of tunnels along the route to cross through the zombie-filled cars and rescue them. The group continues moving towards the safe front cars but inadvertently alert the zombies in the last car before the safe one. Seok-woo and Sang-hwa stay back to hold the doors closed while the others escape, but Yon-suk has convinced Ki-chul and the other passengers that they are potentially infected and they should not help. Sang-hwa sacrifices himself to give Seok-woo time to get the others to safety. In-gil is too slow and is caught by the zombies.Yon-suk and Ki-chul lead the other passengers in the front cars to force Seok-woo's group into a closed vestibule, believing them infected. Jong-gil, shaken by the passengers' selfishness, lets the zombies into the front car, killing the other survivors, though Yon-suk and Ki-chul manage to escape into a restroom. The train is forced to stop near east daegu train station due to a blocked track, forcing the survivors to seek a new train. Yon-suk, in his escape, pushes Ki-chul, Jin-hee and the captain into a pack of zombies; Yong-guk stays with Jin-hee until she reanimates and kills him. An out-of-control train causes a derailment, trapping Seok-woo, Su-an, Seong-kyeong, and the homeless man under a train car. The homeless man sacrifices himself to hold back a zombie horde while the others escape out a small hole, finding a working locomotive. As they head toward Busan, a zombiefied Yon-suk attacks Seok-woo and bites him before Seok-woo can throw him from the engine. Knowing he will turn, he quickly teaches Seong-kyeong how to operate the train and says his fairwells to his sobbing daughter before throwing himself off the engine.
Seong-kyeong and Su-an are forced to stop at a blocked tunnel just before Busan and walk. At the tunnel's exit, military snipers prepare to fire until they hear Su-an sob singing Aloha Oe.
They then realize that the pair are human, and quickly help them to safety.