Sixteen-year-old Mitsu Chikusa attends an industrial high school as an architecture student—or, at least, that is what she has been told by others. When she fell headfirst down a flight of stairs, the resulting injury gave her amnesia, leaving her unable to recall much of her past. After being discharged from the hospital, she returns to school, where she is greeted by her friends and Hideto Hongou—her boyfriend. Although there are a lot of gaps in her memories, Mitsu is relieved to know that someone as reliable as Hideto is close to her. But as she begins to dig into her past, she stumbles upon a note she had written: "As I thought, I like A." This confuses her, as the only people in her life with a name starting with A are her childhood friend and next-door neighbor, Akashi Kurokawa; and her chemistry teacher, Akaike. The note causes Mitsu to question why she was in a relationship with Hideto if her past self did not have feelings for him. To make things more complicated, Hideto seems to be hiding something from Mitsu. As she continues to delve deeper into her past, Mitsu becomes fixated on demystifying the identity of A. [Written by MAL Rewrite] Included one-shot: Volume 2: Nemureru Shishi ni Ai no Koe