In the year 1966, a dark saga begins with Saya, a highly skilled assassin, on a relentless hunt for Chiropterans—vampiric creatures resembling bats with an unquenchable thirst for human blood. Her perilous venture kicks off aboard a bustling subway train, where she swiftly eliminates an unsuspecting man in a suit with tragic accuracy. When her American handlers, David and Louis, arrive to provide her with the details of her next mission, they quickly realize that her latest victim was not the fearsome Chiropteran they had believed him to be.
Saya's new assignment takes her to the Yokota Air Base, where the echoes of impending war with Vietnam threaten to unleash chaos. A rogue Chiropteran has infiltrated the base, and her mission is clear: it must be taken out before it could feed, hibernate, and become an indiscernible threat. To accomplish this, Saya must disguise herself as a schoolgirl and delve into the nearby high school, meticulously tracking the creatures and eliminating them one by one.
As she begins to adapt to her new persona, Saya crosses paths with Amino Makiho, a timid nurse, just before the school's annual Halloween festivities. The night's serenity is violently disrupted when Makiho receives an unexpected visit from her classmates, Sharon and Linda, in her office. Out of the blue, Saya bursts in, her sword gleaming, as she takes down Linda and leaves Sharon critically injured; both girls are ultimately revealed to be Chiropterans, leaving Makiho in a state of disbelief.
At the same time, yet another more sinister Chiropteran moves towards the air base, causing panic to emerge within the school. Makiho regains her senses and follows Sharon into a room filled with revelers in Halloween attire, only to witness Sharon's horrifying transformation. Just in time, Saya arrives to save Makiho, and they scramble to safety in the motor pool, where the Chiropterans corner them.
As tension escalates, David provides Saya with a new sword, which she adeptly uses to eliminate Sharon. However, the final Chiropteran fights back desperately, trying to sneak away on a departing cargo plane. David and Saya pursue it, and after a fierce battle, Saya manages to inflict a mortal wound on the creature. In a chilling and symbolic act, she allows it to drink her blood as it succumbs to its injury. Louis arrives just in time to assist Makiho, as local authorities begin to surround the scene.
As dawn breaks over the chaos of the night, Makiho finds herself under scrutiny in front of government officials during a tense interrogation. Every detail of her account is examined meticulously; however, it becomes increasingly evident that any tangible proof of Saya's daunting fight against the Chiropterans has been purposefully erased from the record. The absence of David and Saya further muddies the waters, leaving Makiho with a narrative shrouded in mystery.
In the course of the questioning, Makiho is shown a photograph intended to jog her memory. Instead, she is confronted with a girl who bears an eerie likeness to herself. The caption reads simply “VAMPIRE,” with an unsettling date inscribed: 1892, sending Makiho's thoughts spiraling with the weight of revelations.
Back at the school, Makiho is left with profound questions surrounding Saya and the Chiropterans. Though she might never fully grasp the truth behind their enigmatic battle, uncertainty lingers in her mind—the idea that Saya may still be out there, bravely fighting against the shadowy forces that threaten to engulf them all.
