Martial law is declared when a mysterious viral outbreak pushes Korea into a state of emergency. Those on an express train to Busan, a city that has successfully fended off the viral outbreak, must fight for their own survival…

Three teenage boys who were once close friends grow apart when they reach high school: Yong-ju lives hiding his true gender identity, Gi-woong becomes the leader of the school gang, and Gi-taek an obsessive manga fan. Tired of the constant malicious bullying by Gi-woong’s gang, Gi-taek betrays them by disclosing that Yong-ju has loved Gi-woong for years. Surrounded by an insecure boundary of the school and wounded by betrayals, the boys are driven into catastrophe.