Plot
The story is set in 1966. Its main protagonist is a girl named Saya, who hunts bat-like vampiric creatures called Chiropterans. Saya is introduced on a subway train, where she assassinates a man in a suit. Her American contacts or handlers arrive. One of them, David, begins to brief Saya on another mission, while the other, Louis, discovers that the man Saya has just killed was probably not a Chiropteran.
Saya's next mission begins at the American Yokota Air Base, which is active in the buildup to the Vietnam War. At least one Chiropteran has managed to infiltrate the air base, and it is only a matter of time before they feed again, go into hibernation, and become untraceable. Saya is to pose as a school girl, infiltrate the high school adjacent to the base, and then track and kill the Chiropterans.
At the school, Saya runs into a meek nurse, Amino Makiho, on the eve of the school's annual Halloween party. Two of Saya's classmates, Sharon and Linda, make a visit to Makiho at the nurse's office. Suddenly, Saya bursts into the room, killing Linda and wounding Sharon, breaking her sword in the process. Both girls are revealed to be Chiropterans. Makiho goes into shock at the revelation. Meanwhile, a third Chiropteran reveals itself and begins making its way to the base. Back at the school, Makiho regains her nerve and pursues Sharon into a room full of dancing Americans in costume, where she finds Sharon transformed. Saya saves her and both flee into a nearby motor pool. The Chiropterans trap them inside and attack.
David delivers a new sword, and Saya uses it to kill Sharon. The final Chiropteran then decides to flee, attempting to stow away on a departing cargo plane. David and Saya give chase and she manages to strike the Chiropteran and mortally wound it. She then stands over the dying creature and lets some of her blood trickle into its mouth. Louis arrives and recovers Makiho before the local police reach her.
Afterward, Makiho is seen in an interview with government officials who question her about the night's events. However, it's revealed that all evidence of the battle between Saya and the Chiropterans has been covered up and both David and Saya have disappeared, leaving Makiho with nothing to prove the veracity of her story. Her interviewer then asks her to identify Saya in a picture which has a girl that looks identical to her, except the picture was taken in 1892. The only other description of the picture is the word "VAMPIRE". Makiho then returns to the school, where she narrates that she never really discovered the full truth behind Saya and the Chiropterans, and wonders if she's still out there fighting them.
