"All of Us Are Dead" opens hours before the outbreak starts.
Nam On-jo (Park Jihoo) and Lee Cheong-san (Yoon Chan-Young) are childhood friends studying at the same school in the same class. Despite knowing each for so long, the two bickers a lot. It turns out that Cheong-san is only teasing On-jo because he has a crush on her. Unfortunately, On-jo has already been drawn to the kindhearted Lee Soo-hyuk (Park Solomon).
Soo-hyuk was not known for his intellect and he was always involved in school brawls back in eighth grade. He currently tries to turn a new leaf by not getting into any more trouble anymore.
Soo-hyuk always has a crush on the quiet and smart Choi Nam-ra (Cho Yi-Hyun). Her classmates call her "prez" since she’s their class’s president, but none of them really respects her. She doesn't have any friends because her dismissive personality and strong demeanor intimidate the other students.
Anyway, their science teacher is the uncanny Lee Byeong-chan (Kim Byung-chul). Despite being a high school teacher, Byeong-chan is brilliant enough to create a virus that transmutes fear and aggression. The goal of the so-called "Jonas Virus" is to give his son the courage to stand up to bullies. However, after injecting it into him, he suddenly turned into a mindless and bloodthirsty monster.
Teacher Lee realized that the virus needed to be studied further, so he experimented with it on a few lab rats. Until one of his students accidentally got bitten by it. Soon after, the other students and faculty members got infected and began to transform into savage zombies. As the virus starts to spread and develop, Nam On-jo and her classmates find themselves trapped in school with nowhere to go.