"The Case Study of Vanitas" is set in a steampunk-inspired 19th-century Paris, where vampires coexist with humans. The narrative centers on Vanitas, a human who wields the mythical Book of Vanitas, a grimoire said to cure vampires afflicted by a curse that distorts their true nature. Vanitas claims to use the book to save vampires, though his methods and motives are often ambiguous. His journey intertwines with Noé Archiviste, a young vampire who seeks the Book of Vanitas to uncover the truth behind its powers and the curse.

The story begins with Noé arriving in Paris to investigate the Book of Vanitas, only to encounter Vanitas himself. Despite initial distrust, Noé becomes entangled in Vanitas’ mission to heal cursed vampires, known as "Charlatans," whose true names have been corrupted, causing them to lose control and act against their will. The duo forms an uneasy alliance, traveling across Paris and beyond to confront various supernatural threats, uncover secrets about the curse, and navigate the complex politics between humans and vampires.

Key characters include Jeanne, a powerful vampire knight bound by a curse, and Dominique de Sade, Noé’s childhood friend with her own hidden agenda. The series delves into themes of identity, morality, and the nature of curses, exploring the blurred lines between good and evil. The narrative is driven by intricate world-building, with factions like the Church and vampire aristocracy playing significant roles in the unfolding drama.

As the story progresses, Vanitas’ past is gradually revealed, shedding light on his connection to the original Vanitas, a vampire who created the book. The series balances action, mystery, and emotional depth, with each arc introducing new challenges and revelations that deepen the central mystery of the curse and the true purpose of the Book of Vanitas. The dynamic between Vanitas and Noé evolves as they confront their own beliefs and the consequences of their actions, making their partnership a cornerstone of the narrative.