A vampire named Saya, who is part of covert government agency that hunts and destroys demons in a post-WWII Japan, is inserted in a military school to discover which one of her classmates is a demon in disguise.

Onigen: You think the more demons you kill the more human you become, don't you? How naïve.
Michael Harrison: Well, you know the old saying; "When the hungry demons feed without discretion..."
Saya: "Onegin is near."