Two weeks before the theatrical release of the feature film adaptation of Kengo Hanazawa's zombie survival manga I am a Hero in Japan, the prequel web drama titled I Am a Hero: The Day It Began (Hajimari no Hi) begins streaming on the NTT DoCoMo service dTV. The story shifts focus away from the manga's protagonist, Hideo Suzuki, to center on the character of Tsugumi "Yabu" Oda, a pragmatic former nurse who becomes a key ally in the main film.

Set during the initial hours of the ZQN pandemic as society collapses across Japan, the narrative follows Yabu as she navigates the early chaos. Using her medical expertise, she assesses the infected and witnesses firsthand the terrifying speed at which the mysterious virus spreads through the population. The plot delves into her experience as a healthcare professional on the front lines, explaining the mechanics of the outbreak before she eventually takes refuge with a group of survivors at the Gotemba Premium Outlets. This setting directly leads into the events of the feature film, where she is depicted as being trapped in a desperate situation, used as a sex slave by the male members of the group, before escaping with Hideo and the high school girl Hiromi Hayakari. Through her eyes, the prequel provides a ground-level view of the nationwide crisis, building the apocalyptic tension that defines the main storyline.