Seok-woo, a divorced fund manager, is an absentee father to his young daughter, Su-an. For her birthday the next day, she wishes for her father to take her to Busan to see her mother.
oh no father
They board at Gwangmyeong Station. Others on the same train are tough working-class husband Sang-hwa and his wife Seong-kyeong, rich but selfish CEO Yon-suk, elderly sisters In-gil and Jon-gil, and a homeless man who is experiencing post-traumatic stress disorder, and as the train departs, a young woman boards the train with a bite wound on her leg. The woman soon becomes a zombie and attacks a train attendant, who then also turns into a zombie.
we are "zombies"
A blocked track at East Daegu train station forces the survivors to stop and search for another train.
Su-an and Seong-kyeong escape with Seok-woo into the train that had been activated. They encounter Yon-suk in the engine room, on the verge of turning into a zombie, having been bitten when the train conductor saved him. Seok-woo fights him off, but is himself bitten. He puts Su-an and Seong-kyeong inside the engine room and shares his last words with his daughter before moving outside. As he zombifies, he thinks of the first time he held his daughter in his arms, and throws himself off the locomotive with a smile.
Su-an and Seong-kyeong get off at Busan and begin walking through a train tunnel. On the other side of the tunnel are soldiers stationed to defend the perimeter against zombies. Unable to see the new arrivals clearly, the soldiers at the checkpoint are instructed to shoot them.
The soldiers then hear singing, which makes them realize the newcomers are human: it is Su-an, tearfully singing the song that she had wanted to perform for her father at the beginning of the film, now singing it to honor his heroic sacrifice.