"The Case Study of Vanitas" is set in a steampunk-inspired 19th-century Paris, where vampires coexist with humans. The narrative centers on Noé Archiviste, a young vampire traveling to the city in search of the Book of Vanitas, a legendary grimoire said to hold the power to either save or doom vampires. Upon arriving, Noé encounters Vanitas, a human who claims to be a doctor specializing in vampires and the self-proclaimed bearer of the Book of Vanitas. The book, however, is no ordinary artifact—it is cursed and tied to the enigmatic Vampire of the Blue Moon, a figure shrouded in myth and tragedy.

Vanitas wields the book to heal vampires afflicted by malnomen, a condition that distorts their true nature and drives them to act against their will. Despite his noble intentions, Vanitas is a morally ambiguous character, driven by a personal vendetta and a desire to defy the curse of the Blue Moon. Noé, intrigued by Vanitas's methods and motives, decides to accompany him, forming an uneasy partnership. Together, they navigate a world of intrigue, uncovering the secrets of the Book of Vanitas while confronting powerful factions within vampire society, including the Charlatan, a shadowy organization with its own agenda.

The series explores themes of identity, free will, and the nature of curses, weaving a complex narrative that delves into the pasts of both protagonists. Noé’s loyalty and curiosity often clash with Vanitas’s cynicism and hidden pain, creating a dynamic relationship that evolves as the story progresses. Supporting characters, such as Jeanne, a vampire knight, and Dominique, Noé’s childhood friend, add depth to the narrative, each carrying their own burdens and connections to the central conflict.

As the plot unfolds, the duo faces numerous adversaries, including other vampires, humans, and entities tied to the Book of Vanitas. The series balances action, mystery, and emotional depth, with each arc revealing more about the characters’ motivations and the world they inhabit. The interplay between Noé and Vanitas, along with the intricate lore surrounding vampires and the Book of Vanitas, drives the story forward, culminating in a tale that is both fantastical and deeply human.