A truck drives into Jinyang, where it is immediately met with a quarantine checkpoint. As personnel stop and disinfect the vehicle, the driver asks them what is happening; One of the workers explains that there was a leak in the biotech district, but that it shouldn’t pose any major problems, and lets him pass. As the man drives, his phone rings, and as he attempts to pick it up, he loses concentration and accidentally hits a deer that stumbled onto the road. He checks the truck for any damage before leaving, only for the deer to suddenly reanimate and stand up, its eyes foggy…
Seok-woo is a divorced, workaholic hedge fund manager who is currently at work, discussing the sale of certain stocks on the phone; As he browses the Internet on his computer, he comes across a web article detailing how all the fish in a reservoir have mysteriously died. When his assistant comes in, Seok-woo tells him to sell all the stocks from that company. Later that night, he returns home while arguing with his ex-wife about their daughter, six-year old Soo-an – Her birthday is coming up, and she wants to go to Busan and visit her mother, but Seok-woo is too busy with work and doesn’t want her to go by herself. At home, Seok-woo finds Soo-an secretly talking to her mother on a video call, and explains that they can visit Busan in a week once he has less work – he gives her a Wii as a birthday present, but Soo-an is dismayed, having received the same present for her previous birthday. She tells her father that all she wants for her birthday is to see her mother, and that she’ll go by herself so she won’t burden him.
Seok-woo has a private conversation with his mother about Soo-an; She tells him that he should go to Busan now and have a heart-to-heart with his ex-wife, telling him that while he may be very busy, this is a very important time in Soo-an’s life. She then shows Seok-woo a video of his daughter’s recital at school; While Soo-an attempted to sing “Aloha ‘Oe”, she suffered stage fright because Seok-woo wasn’t there for her. Realizing how badly he neglects Soo-an, Seok-woo changes his mind and decides to take Soo-an to Busan on a KTX train; As they drive to the station in Sheol, Seok-woo attempts to make a life lesson out of the recital, but nearly hits an ambulance speeding past him on a crossroads; The ambulance is followed by an entire convoy of emergency vehicles, and Soo-an notes that a skyscraper in the distance is on fire, causing ash to float down from the sky.
At the station, KTX-101 is preparing to depart. As Conductor Seok-yong makes final preparations, the passengers board the train, including Seok-woo and Soo-chan, a baseball team heading to an away game (who cheer when classmate Kim Jin-hee arrives, having signed on as cheerleader to see her crush Min Young-guk), and elderly sisters Jong-gil and In-gil. As the train leaves, however, an injured woman runs inside before the doors close to take shelter, unnoticed by staff; Soo-an also sees a station attendant being tackled by a man. The injured woman, who has a bite wound on her right leg, attempts to tend to her injury in a toilet, only to see her surrounding veins starting to turn black.
Young-suk, a COO of the Stallion Express cooperation, alerts one of the train attendants that he saw someone jump onto the train and hide in the toilet – Soo-an joins them, curious. The attendant unlocks the toilet, but it is not the woman – rather, it is a terrified homeless man, who keeps muttering how everyone back at the station is dead. As the attendant tries to calm him down, Young-suk tells Soo-an that she’ll end up like the homeless man if she doesn’t do well at school; She rebuffs him, stating that her mother said anyone who says such things is a bad person. Soo-an attempts to visit the toilet, but runs into a man named Yoon Sang-hwa, who tells him that his pregnant wife Seong-kyeong is using the toilet and that she’ll have to use one further down the train. Meanwhile, the stowaway woman staggers through the train, her veins becoming increasingly tainted unbeknownst to the passengers, before she collapses and faints in the back of the train. A female attendant, Min-ji, discovers the woman convulsing and shaking, and radios for medical assistance.
Seok-woo is awoken from his slumber by his phone ringing, and answers it to find one of his colleagues on the phone; He panics, and warns Seok-woo of violent civic unrest across the country. As the passengers watch the news, they see reports of violent riots breaking out across South Korea; Seok-woo then notices his daughter is missing, and goes to look for her. The stowaway woman reanimates, now a zombie, just as Min-ji turns her back; She then attacks, infecting Min-ji, several of the baseball players, and other civilians. Chaos ensues aboard the train, with the chief attendant urging passengers towards the front, only to be caught and bitten by Min-ji while alerting a sleeping woman. Seok-woo and Soo-an witness the zombies charging across the train, and flee to the front; Sang-hwa and Seong-kyeong also see the carnage, and the former fends off a zombie that attacks them and infects a woman.
As the remaining passengers reach Car 8, Young-suk orders Seok-woo to block the door to stop the zombies getting any further; He does so, only to lock Sang-hwa and Seong-kyeong outside as they were sealing the zombie in the toilet. Fortunately, Sang-hwa overpowers Seok-woo and opens the door, enabling him and his wife to get inside and lock the door just as the horde reaches them; Upon further observation, the passengers discover that the zombies have lost their higher brain functions, being unable to open the unlocked door – Young-suk then covers the door with a paper mâché of newspapers, causing the zombies to calm down. Sang-hwa calls Seok-woo out for leaving him and his wife to die, to which Seok-woo replies that the lives of everyone who was already in safety mattered the most.
Conductor Seok-yong announces over the intercom that due to orders from the control station, KTX-101 will not be stopping at the next city. Seok-woo then receives a phone call from his mother, who tells him that everything back home is in chaos; She begs him to take care of Soo-an, before turning into a zombie over the phone – Seok-woo chooses not to tell Soo-an of his mother’s infection. As the train passes through the next station, the passengers see that the city has already become overrun, with passengers begging for help and banging on the windows before being attacked and infected by zombies. A government official on the news explains how violent riots are breaking out in major South Korean cities, and that many major districts have been quarantined. The survivors check their phones to find that videos of the riots, along with “#ZOMBIES”, are trending on social media. The official declares a nationwide state of emergency, and orders civilians to quarantine themselves in their homes and not to trust any unverified news. Despite assurances that everything is under control, Seoul has descended into anarchy.
As the passengers settle in for the ongoing journey, Soo-an tells Seok-woo that she can’t contact his mother; As he attempts to call her again, Soo-an gives up her seat for In-gil – Seok-woo tells her not to be altruistic, as people need to look out for themselves above all. The conductor then announces that the train will be stopping at Daejeon, where a military checkpoint has secured the station and will take the citizens to safety; However, as Young-suk is making a phone call to check which cities remain safe, Daejeon is not on the list; Seok-woo overhears him, and phones a colleague. Upon hearing that they’ll be quarantined, he offers to help the colleague in the future if he and Soo-an can evade quarantine; The colleague agrees, and instructs them to head to the east square when the train arrives, where some of the military will be waiting for them. Meanwhile, Soo-an talks with Sang-hwa and Seong-kyeong, where she apologizes for her father’s behaviour and explains his job as a hedge fund manager. Sang-hwa shows open disdain for Seok-woo and calls him a bloodsucker, which Soo-an doesn’t defend him against.
The train arrives at Daejeon station, which is completely abandoned. The passengers disembark and head for the main square, but Young-suk pulls both Seok-yong and train attendant Ki-chul aside; He informs them that Daejeon is not safe, and that their main objective should be to head to Busan, which is one of the last surviving cities in South Korea. He demands that the compromised cars be disconnected from the KTX, but Seok-yong states that it cannot be done. The three decide to head for Busan, but the conductor insists that they won’t depart until the survivors have returned to the train. Inside the station, Seok-woo and Soo-an separate from the others to head to the east square, with the homeless man, having overhead Seok-woo’s phone call, following them.
When Soo-an finds out what’s going on, she attempts to inform the other passengers, only for Seok-woo to stop her. He reminds her again to look after herself, but Soo-an tearfully calls him out for being selfish, stating that he cares about work more than family and blaming him for her mother’s divorce. As the main group of survivors descend an escalator to head outside, they discover that Daejeon has long fallen to the infection, with the infected military soldiers waiting outside – they attack, infecting the passengers on the escalators who cannot climb up in time, but Sang-hwa lifts himself and Seong-kyeong onto the main stairs so they can escape; At the same time, Seok-woo and the homeless man discover the last surviving soldier, who is quickly attacked and mauled by his former comrades.
As the two make a dash back to the station, Sang-hwa and the other passengers arrive in the main area and save Soo-an from being bitten from behind; As Soo-an flees to the train alongside Seong-kyeong, Seok-woo is tackled by a zombie; The homeless man saves him, and the two manage to reach a door as Sang-hwa, Jin-hee and the baseball team hold it open for them; The men desperately try to barricade the door as the others go ahead. As the women reach the train, they notice some survivors dying after they accidentally attempt to flee into the compromised cars, and are forced to take the other side of the train – shortly afterwards, the zombies manage to break through the windows on the upper floors and collapse onto the train roof, landing on the station and dividing the survivors. Soo-an, In-gil, Seong-kyeong, and the homeless man are separated from the others and forced to take shelter in the back of the train, while Jong-gil and Jin-hee manage to reach the front.
With the train surrounded by zombies, Young-suk orders Ki-chul to order the conductor to depart; Jin-gil objects, stating that they would be condemning Seok-woo, Sang-hwa, and the baseball team to die, but Young-suk silences her and uses his status to make Ki-chul comply. Back in the station, the men lock the door and flee for the station just as the zombies break through the glass doors; Two of the baseball players fall behind and are caught by the zombies, but Seok-woo, Min Young-guk, and Sang-hwa manage to board a central car and survive, the latter also obtaining some riot gear left on the station. As the passengers evaluate the situation, Seok-young contacts the control station and is ordered to head for Busan. The survivors are split into three groups; Seok-woo, Min Young-guk, and Sang-hwa are trapped in Car 9, the homeless man and the separated woman have barricaded themselves in Car 13’s toilet with zombies trying to break in, and the others remain at the front of the train.
As the passengers at the front car use fire extinguishers to obscure the door again, Min Young-guk calls Jin-hee and reveals that they are the last surviving members of the baseball team, having been traumatized by the deaths of his friends. Seok-woo, inspired by the kindness he was shown and Soo-an’s words, decides to look out for others more, and he and the other men agree to rescue the women, arming themselves with weapons. The group manages to fight through two cars before being overwhelmed in the third, but before anyone is bitten, the train passes through a tunnel, causing the zombies to immediately calm down; Realizing the zombies only attack if they can see their targets, Seok-woo lures the zombies past him by tapping a baseball bat on the luggage rack, and the men use their phones as noisemakers to distract the zombies. They successfully rescue their loved ones from the toilet, and have a tearful reunion.
Min Young-guk texts Jin-hee, informing her that they are alive and will wait until the train reaches the next tunnel before heading back. She is excited and lets the other know, but Young-suk becomes paranoid that they might be infected and refuses to let them in; Having used his status to establish himself as leader of the survivors, the other civilians agree with him, leaving Jin-hee outnumbered. The survivors in the back begin crawling through the luggage racks to get past the zombies, but the train exits the tunnel and the homeless man accidentally steps on an empty can of soda, alerting the zombies. The group attempt to block the door, but a zombie jams it at the last moment, and the men desperately try to keep it closed. Young-suk has the passengers barricade the door from their side and keep Jin-hee hostage to stop her from helping, leaving the survivors trapped. Min Young-guk eventually smashes the door down with a baseball bat, but Sang-hwa is bitten and Young-suk desperately holds the next door shut.
Jin-hee and Jong-gil attempt to stop the other passengers and help the survivors, as Sang-hwa orders the others to leave him; Understanding that he will turn, he accepts his fate and bids goodbye to his wife, telling her the name for their unborn child just as the door breaks; The other survivors run for safety, with Sang-hwa holding the horde back for several seconds before succumbing to the infection; However, In-gil doesn’t make it in time, and is devoured by the horde. Seok-woo furiously calls out Young-suk for his selfish actions, but he screams that they are infected and demands the survivors isolate themselves in the vestibule; With all the passengers siding against them, they reluctantly comply – Jin-hee joins them, disgusted by Young-suk. Seeing her infected sister on the other side of the door and Young-suk’s actions, Jong-gil loses her faith in humanity and commits suicide by opening the door, before anyone can stop her – the zombies break in, and while Young-guk and Ki-chul seal themselves in the toilet, the other passengers are killed and infected.
Seok-woo talks to Soo-an, apologizing for his past behavior and promising that she will survive. He receives a call from his colleague, who informs him that Busan has become a stronghold against the zombie apocalypse; He also reveals that the zombie virus came from the biotech company they bought. Seok-woo grieves, believing he is responsible for the apocalypse. KTX-101 finally arrives at the next station, which has been blockaded by crashed train cars; Seok-yong explains to the passengers that he is going to head out into the open and find a new train that they can board. As the remaining survivors head out to join him, Seok-yong manages to locate a train and gets it going; Young-suk escapes the train after forcing Ki-chul into the horde, killing him, but another train crashes into KTX-101 and topples it sideways, separating the survivors. Young-suk then leaves Min Young-guk and Jin-hee to the zombies; They fend them off, but Jin-hee is bitten – with all his friends dead, Min Young-guk falls into despair and loses the will to live. When Jin-hee turns in his arms, he doesn’t defend herself, and she devours her former crush.
Soo-an wakes an unconscious Seok-woo, who realizes that they, Seong-kyeong, and the homeless man are trapped beneath the toppled car, with zombies trying to break through the windows above them. Young-suk is rescued from zombies by Seok-yong, but he immediately betrays the conductor and leaves him to be mauled by a zombie. Meanwhile, Seok-woo and the girls manage to escape from under the train, but the zombies break through the windows, forcing the homeless man to sacrifice himself so they can keep going. The three run for their lives and manage to catch the train that Seok-yong started, but the zombies start to cling onto the back, forming a heavy mass that slows the train down. Seok-woo kicks the zombies off, but they find that Young-suk, who also made it onto the train, was bitten despite hie best efforts – he turns and attacks them, managing to bite Seok-woo before the three force him off of the train.
Seok-woo forces both Soo-an and Seong-kyeong into the conductor’s seat, and explains to them how to stop the train at Busan, bidding goodbye to his daughter before resisting the infection long enough to lock himself outside. As he turns, he hallucinates, remembering the happiest moment of his life – cradling a newborn Soo-an – and tearfully smiles, falling off of the train to his death. Soo-an and Seong-kyeong arrive in Busan, navigating through the demolished station and into a tunnel; On the other side, military are keeping watch for infected. A sniper sees their silhouettes and believes they are infected, but before he fires, Soo-an begins to sing “Aloha ‘Oe” in remembrance of her father, having forgiven him for his past behavior. Realizing they are survivors, the military hold their fire – Soo-an and Seong-kyeong are then rescued, having survived the train to Busan.
