The story is set in the American Yokota Air Base located in post-WWII Japan, a few months before the beginning of the Vietnam War. Its main protagonist is a girl named Saya, who hunts hematophagous bat-like creatures called Chiroptera for a secret organization known as Red Shield.

side note:
Blood Plus (Blood +) the series is based on the film.

Part 1

Part 2

Part 3

Part 4