Teropterids, a sinister breed of creatures, have the deceptive capability of imitating human beings with the sole aim of satisfying their insatiable craving for human blood. However, mankind has put in place special squads tasked with exterminating these malevolent beings. Among the highly skilled assassins is a mysterious and thoughtful girl known as Saya, who stands out in her craft. In Japan’s 1960s, Saya is sent to probe a U.S. military base believed to be infested with these repugnant creatures.