The series centers on Reiko Houshou, the young heiress to the vast Houshou Group global enterprise, who lives a double life as a rookie detective with the Metropolitan Police Department. To hide her privileged background from her colleagues, she dresses in plain suits and glasses. Her direct superior is Inspector Kyouichirou Kazamatsuri, the flamboyant heir to the Kazamatsuri Motors fortune, whose confidence in his own detective skills far exceeds his actual abilities. While both come from wealthy families, their professional results are unremarkable.

The narrative's core dynamic shifts when Reiko returns home each evening to her luxurious mansion. Shedding her work clothes for a dress, she discusses the day's most difficult cases with her mysterious and sharp-tongued butler, Kageyama. During dinner or after, Kageyama listens to the details Reiko provides. Although he speaks with impeccable politeness, he does not hesitate to deliver scathing critiques, often questioning whether her eyes are merely for decoration. He then proceeds to unravel the entire mystery from his armchair, using pure logic and the smallest overlooked details Reiko provides, refusing to engage in field work he considers the police's job.

The setting alternates between crime scenes across Japan and the opulent Houshou dining room, where the true deductions occur. The central conflict arises from Reiko's need to prove herself as a detective while being utterly dependent on her condescending butler, whose real name and past remain a complete secret. Kageyama claims he once wanted to be a detective or a professional baseball player, and he often uses baseball metaphors in his explanations.

Several notable narrative arcs define the series. An early case introduces the formula when a murder occurs at a birthday party for Reiko's uncle, with her own college friends becoming the primary suspects. With only twenty minutes to identify the culprit, Kageyama appears to solve the case, establishing their working relationship. Another complex investigation involves the murder of a real estate tycoon, where Reiko must analyze an erased dying message and a bloody trophy, learning to think inside the victim's mindset to catch a murderer driven by a chain of revenge. A locked-room mystery at a famous painter's studio forces Reiko to confront a deranged suspect, while Kageyama locates a hidden passageway to solve the seemingly impossible escape. Later cases venture into modern settings, including a death connected to a popular VTuber where online fan outrage complicates the investigation, and a dating app scam where the murderer uses elevator shoes to disguise their height. Personal stakes rise when Reiko attends the wedding of a woman she cherishes like a sister; the bride is attacked in a locked room, and Reiko herself becomes suspected as an accomplice, risking exposure of her secret identity. A subsequent arc involves a 50 million yen kidnapping ransom, where Reiko and Kageyama pose as a married couple to deliver the money, ultimately connecting the crime to an incident from three years prior.