In the year 1923, Tokyo is engulfed in a vampire crisis. The illicit trafficking of "Ascra," an artificial blood supply, causes the vampire population to explode within the metropolis. The Special Forces Force 16 recruits vampires within the military under instructions from Lieutenant General Sounosuke Nakajima to form Code Zero, a new clandestine vampire-hunting unit. Colonel Yoshinobu Maeda commands the unit. He is in charge of identifying the source of the Ascra trade, as well as arresting and murdering vampires, and he is under pressure to provide fruitful results in the face of mounting criticism that threatens to decommission the unit. To make matters worse, the vampires in the unit are trying to understand their newfound powers and identities now that they are no longer under the control of the vampires.