In the year 1923, a vampire crisis takes Tokyo by storm. Provoked by the illegal trade of "Ascra," an artificial blood source, the population of vampires begins to rapidly increase within the city. Under orders from Lieutenant General Sounosuke Nakajima, the Special Forces Unit 16 gathers vampires within the military to create a new undercover vampire-hunting unit known as Code Zero. Leading the unit is Colonel Yoshinobu Maeda. Responsible for locating the source of the Ascra trade as well as capturing and killing vampires, he is pressured into producing fruitful results amidst the growing criticism that threatens to decommission the unit. To make matters worse, the vampires within the unit are struggling to grasp their newfound powers and identities now that they are no longer human. With the world at odds against them, the members of Code Zero must find a way to curb the crisis or otherwise fall victim to their own demonic natures.

Losing their war against Heaven's angel army, the demons of Hell are on the cusp of collapse. As a last ditch attempt to turn the tides of the war, the demons send Akutsu Masatora to the mortal plane on Earth to save their homeland from the angels. Disguising himself as a high school student, Akutsu seeks to recruit someone with the capability to lead the demons to victory. Akutsu does not have to look far: among his classmates is the magnificent Lily Amane, a transfer student who seems like the perfect candidate for Akutsu's plans. But as luck would have it, Lily is one of the very angels that Akutsu is meant to be fighting against, and his attempt to recruit her might end up being the biggest blunder in the failing war. Worse yet, Akutsu may fall in love with his own worst enemy.