When Kojou Akatsuki, a regular high school student on Itogami Island's Demon District, gains vampire powers after a chance encounter, his ordinary life takes a dramatic turn. Unveiled as the fourth primogenitor, a legendary powerful vampire, Kojou becomes a target for the Lion King Organization. Tasked with monitoring or eliminating him if necessary, apprentice Yukina Himeragi is sent to oversee Kojou. As they navigate this uneasy partnership, Kojou grapples with his newfound abilities while they join forces to safeguard their city against rising threats.