Reiko Hosho is the heiress to the vast Hosho Group, a world-renowned conglomerate, but she has chosen a life that defies her privileged upbringing. By day, she works as a rookie detective at the Kunitachi Police Department, keeping her true identity a secret from all but a few. Her immediate superior is Inspector Kyoichiro Kazamatsuri, the flashy and ostentatious heir to the Kazamatsuri Motors fortune. Despite his loud confidence, the inspector is largely incompetent, often making grand but illogical pronouncements that do little to solve the cases that land on their desks.

It is at her grand family mansion that Reiko finds the true key to cracking her most baffling cases. Her sharp-tongued butler and driver, a man simply named Kageyama, listens to the details of her investigations while she changes from her work clothes into an elegant dress for dinner. After the meal is finished and the table is set, Kageyama proceeds to dismantle the case. He does so with a brutally condescending wit, often beginning his deductions with a sarcastic jab at Reiko's observational skills, such as asking if her eyes are merely decorative. Though his manner is insufferable, his intellect is undeniable. From a single evening's conversation, he identifies seemingly insignificant details that Reiko overlooked and reconstructs the entire sequence of events, flawlessly identifying the culprit and the motive without ever having visited the crime scene.

The light novel series follows this established episodic formula, with Reiko bringing a new mystery to the dinner table in each chapter or story arc. The cases range from murders at high-society parties to the death of a small business owner, each one a puzzle box that Kageyama unlocks through logical deduction and a deep understanding of human nature. One early case involves the assault of Reiko's college senior at a party, where Kageyama uses the changing color of a gemstone under sunlight to reveal the attacker. Another case deals with the murder of a woman whose cold exterior hides a secret tenderness, revealed only after her death when her family finds a treasured photo album in her safe. A later, more emotional arc follows the death of a kind man who was a devoted fan of a virtual YouTuber, a case that ends with the performer singing a requiem for the viewer who supported her without ever knowing her true identity.

The narrative also develops its recurring cast beyond the central trio. Inspector Kazamatsuri remains a source of comedic relief, his pompous declarations contrasting sharply with Kageyama's quiet, cutting logic. The dynamic between Reiko and Kageyama slowly evolves from one of pure frustration to a begrudging professional dependence, though the butler never softens his critical assessments of her work. The sequel novel series, Shin Nazotoki wa Dinner no Ato de, continues the adventures of the mismatched pair, introducing new cases and further exploring the world of high finance and low crime that they inhabit. Throughout the series, the seemingly frivolous comedy and the ritual of the dinner table serve as a counterpoint to the often tragic and deeply human stories of jealousy, greed, and sorrow that lie at the heart of each mystery.