Fund manager Seok-woo is a cynical workaholic and divorced father of his daughter Su-an, who wants to spend her birthday with her mother in Busan. Seok-woo sees a video of Su-an attempting to sing "Aloha ʻOe" at her singing recital and succumbing to stage fright as a result of his absence. Overcome with guilt, he decides to grant Su-An's birthday wish. The next day, they board the KTX 101 at Seoul Station, en route to Busan. Other passengers include Sang-hwa and his pregnant wife Seong-kyeong, COO Yon-suk, a high school baseball team including Yong-guk and his cheerleader girlfriend Jin-hee, elderly sisters In-gil and Jong-gil, train attendant Ki-chul, and a traumatized homeless stowaway hiding in the bathroom. As the train departs, an ill young woman runs onto the train unnoticed. She turns into a zombie and attacks a train attendant, who also turns. The infection spreads rapidly throughout the train.