In 1923, Tokyo was in the midst of an urgent vampire predicament spurred by the illicit Ascra trade. This led to a rapid surge in the vampire population. In response, Lieutenant General Sounosuke Nakajima directed Special Forces Unit 16 to form a clandestine group known as Code Zero, comprised of military vampires. There, Colonel Yoshinobu Maeda was appointed as the leader of the group and tasked with identifying the source of the Ascra, as well as eliminating vampires, all while being confronted with mounting criticism that threatened to disband the team. Adding to the complexity, the vampire members were struggling to come to terms with their newfound powers and no longer identifying as human. Code Zero needed to overcome the pressures of the world against them and solve the crisis, as the team teetered on the edge of losing themselves to their own demonic nature.