Saint October takes place in a fantastical, horror‑laden world that sits in the shadows of a demon city. Within this city, a series of baffling incidents that defy scientific explanation begin to occur. To investigate these events, three girls with unique powers come together as detectives. Hayama Kotono, an orphan raised in a convent; Shirafuji Natuski, Kotono’s classmate and a pop idol; and Hijiri Misaki, a mysterious figure who confronts nightmares from her past life. As the girls delve deeper, they confront forces that seek to bring Judgment Day upon the world and others who fight to protect it, forcing them to question what truly constitutes justice versus evil. Along the way, Kotono encounters a boy who has lost his memory, and the investigation sets in motion a chain of events that may determine the fate of all.