When Aoi Shirase learned that her brother was killed while in service to the army, only her childhood friend Shuutarou Kurusu was there to comfort her. And so, when Shuutarou is drafted as an escort for dangerous goods, Aoi implores him simply not to die. Yet, despite his vow, she later loses him to the battlefield. Unable to cope with this tragedy, Aoi clings to the hope that Shuutarou is still alive since his body is never returned to her.

Unbeknownst to Aoi, Shuutarou had been attacked and involuntarily transformed into a vampire. He is enlisted into the military's special unit, "Code Zero," alongside fellow monsters who hunt their own kind. With a mission to hunt down any superhuman dangers to Tokyo's populace, Shuutarou is surprised to frequently run into Aoi. For some reason, she is drawn to vampires, both in and out of her job as an investigative reporter.

Unable to reveal his fate, Shuutarou resolves to watch over Aoi from a distance—but is there any point to protecting a loved one whose only wish is never to have been parted?