At Megurigaoka Private High School, a typical school day begins to show small signs of something wrong as an increasing number of students fail to show up for classes. The quiet unease soon explodes into full-scale horror when members of the student body suddenly turn into violent, reanimated corpses and begin attacking their former classmates. In the chaos of this sudden zombie outbreak, a small group of female students manages to escape and barricade themselves inside a secure room of the school, realizing that their only chance of survival is to stay hidden within the building as the world outside collapses around them. They are living through day one of what may become the rest of their lives.

The story centers on a set of four survivors distinct from the main film adaptation. Yukari Sawatari, a third-year student who endures cruel bullying from another student, finds herself forced into an uneasy alliance with her tormentor. Kozue Kubota, a member of the video editing club, was working on a documentary project with a friend when the outbreak hit, only to witness that friend get bitten and turn during the initial panic. Riko Harada, from the acting club, struggles with her perfectionist tendencies which had already strained her relationships with her peers before the undead began roaming the halls. Maho Takagi, the third-year student who had been bullying Yukari, must confront the collapse of her social hierarchy in a world where high school status means nothing against the ravenous infected. Teacher Megumi Sakura, affectionately called Megu-nee by her students, serves as a bridge to the main film's continuity as she attempts to guide students through the nightmare while dealing with her own fears.

The web series functions as an original prequel story that runs parallel to the events of the live-action School-Live! film, depicting the initial outbreak from the perspective of a different group of girls. Over the course of four episodes, the narrative explores how ordinary high school concerns such as friendship disputes, artistic ambitions, and bullying dynamics become matters of life and death when the rules of civilization vanish overnight. The survivors must grapple with immediate practical needs like securing food and water while also facing impossible moral questions, including whether they can bring themselves to destroy former friends and classmates who have turned into zombies. The cramped safe room becomes a pressure cooker for unresolved conflicts and emerging paranoia, especially when it becomes clear that one of the four was bitten and may be hiding the wound from the others. The series provides a darker, more somber look at the first day of the apocalypse, focusing on the slow erosion of hope and the difficult choices that survival demands from ordinary schoolgirls with no special training for the end of the world.