After Armageddon, an apocalyptic war, humanity faces another threat: vampires. Ongoing clashes between the two races have divided the world into distinct factions. The vampire race, known as Methuselah, aligns with the New Human Empire; meanwhile, humans, referred to as Terrans by the vampires, form the Vatican Papal State. Additionally, extremist groups such as the Rosenkreuz Order seek to reignite the war, despite the factions' efforts to prevent direct conflict. To fight terrorist organizations, the Vatican has established the AX unit. Under the leadership of Cardinal Caterina Sforza, AX agents investigate vampire-related incidents, hoping that Terrans and Methuselah will eventually achieve peaceful coexistence. Part of the AX unit is Father Abel Nightroad—a seemingly confused but kind-hearted man who becomes a fierce vampire slayer in battle. He is joined by his partner, Agent Sister Esther Blanchett, a courageous and gentle young nun burdened by a tragic past. As the pair grow closer, they start to uncover evidence of malicious plots and dark forces operating in the shadows. However, their journey is fraught with misfortune that may compel them to face the memories haunting their hearts.