In 1923, a vampire crisis sweeps through Tokyo. Triggered by the illegal trade of "Ascra," an artificial blood source, the city's vampire population begins to surge. Acting on orders from Lieutenant General Sounosuke Nakajima, Special Forces Unit 16 recruits vampires into the military to form a new covert vampire-hunting squad called Code Zero. Colonel Yoshinobu Maeda leads the unit. Tasked with tracking down the source of the Ascra trade and capturing or killing vampires, he faces pressure to deliver results amid mounting criticism that threatens to disband the unit. Compounding the issue, the vampires within the squad struggle to adapt to their new powers and identities, now that they are no longer human. With the world against them, the members of Code Zero must find a way to contain the crisis or succumb to their own demonic natures.