The year is 1923 and vampires live among the humans in Tokyo.
The vampires live in the dark and feed on human blood. They have existed for several centuries, somehow managing to live in hiding until present day.
Within the army, the government and Lieutenant General Nakajima create a special unit, called “Code Zero”, to deal with information war. Both the unit and it's creator are reassigned to solve the vampire crisis.

The members of this unit are...

Yoshinobu Maeda, the best army colonel in the world; Shutaro Kurisu, a newbie to the army but the strongest vampire in the country; Tokuichi Yamagami, who has the brains but not skills; Suwa who has been a vampire ever since the Edo period; and the mad scientist Takeuchi, whose brain never seems to age.
The number of vampires only seems to increase. There are people moving behind the scenes, and they learn of the existence of a mysterious artificial blood source called Ascra.
Deflotte watches all of this unfold very calmly. She is a vampire whose age is well over 300 years, and yet her appearance is that of a child.
Code Zero has to put a stop to all of this before the vampires take over the city!