A truck makes its way towards a toll booth that is currently undergoing sanitization due to a minor leak at a nearby nuclear plant, with several workers dressed in hazmat suits diligently cleaning the area. The truck driver complains about the delay, but the workers assure him everything is fine. As he drives away, distracted by his ringing phone, he accidentally strikes a doe. Upon inspection, he drives off, but the doe manages to rise and ominously reveals its infection.
Meanwhile, in Seoul, we meet Seok-Woo, a dedicated fund manager whose work takes precedence over his daughter, Su-an, and even his divorce. After a long, stressful day and a poorly timed birthday gift for Su-an, she expresses her wish to visit her mother in Busan. Initially, Seok-Woo hesitates, but his mother’s video of Su-an singing Aloha 'Oe at a school recital—one he missed attending—compels him to book a ticket on the next KTX train to Busan.
On the morning of their departure, Seok-woo drives Su-an to the station, where they nearly collide with an approaching horde of ambulances and police vehicles. Su-an’s curiosity leads her to catch a burning ember outside the window, drawing Seok-Woo’s attention to a distant burning building. Ignoring the warning signs, they board the train, which is filled with a diverse group of passengers, including Sang-hwa, a tough-working husband, and his pregnant wife Seong-kyeong, a high school baseball team, the self-centered CEO Yon-suk, along with two elderly sisters, In-gil and Jong-gil, and a homeless man who seems to have an inkling of the chaos unfolding.
As the train prepares to depart, a frantic young woman with a massive bite wound stumbles on board. The station manager signals the train to leave, but catches sight of people at the top of the stairs, screaming in terror. Su-an observes as the manager is swiftly ambushed by a rabid human, a sight that terrifies her. Seeking refuge, she heads to the bathroom, while chaos erupts in the lower compartments as the infected woman transforms, spreading the infection to others, particularly targeting the baseball team.
With the zombies wreaking havoc, panic ensues as they quickly make their way toward the upper levels. Seok-woo awakens to find Su-an missing, and a frantic call from a coworker informs him of violent outbreaks occurring throughout Korea. In a desperate search for his daughter, Seok-woo discovers Su-an face to face with an advancing horde of zombies, prompting an intense escape back to their compartment.
The survivors rally to barricade themselves within the train, realizing that the zombies do not comprehend how to open doors. As they strategize, Seong-kyeong ingeniously uses water and newspapers to obscure the windows and create a semblance of safety. The conductor’s announcement sends the train toward Daegu station, where they have been informed military forces will secure the area. However, Yon-suk's true nature soon reveals itself as he manipulates the other passengers to bolster his own survival.
As the train reaches Daegu, the survivors’ plight intensifies. They encounter increasing hordes of infected soldiers, faced with terrifying choices to escape or fight. Seok-woo and his group navigate their way toward safety, leading to heart-wrenching decisions and losses, revealing the depths of human despair amidst chaos.
When they make their way to a cargo train, Seok-woo confronts Yon-suk, who has evaded the onslaught. Betrayals ensue as Yon-suk sacrifices others in his scramble for survival. The action reaches a climax with the heartbreaking separation of Su-an, Seong-kyeong, and Seok-woo, leading to a devastating sacrifice by the homeless man.
In an emotional farewell, Seok-woo realizes his infection takes hold, compelling him to protect his daughter one last time. As the train approaches Busan, a sniper stands ready to eliminate any potential threats, yet Su-an’s poignant rendition of Aloha 'Oe stops them in their tracks, showcasing the strength of love and hope amidst helplessness as they race to her side, signaling that even in darkness, humanity may prevail.
