In 1932, the seventh year of the Showa era, the imperial capital Tokyo begins to echo with the footfalls of an uneasy age. Amid this atmosphere of growing tension, Eiko, a young lady from an upper-class samurai family, shares a unique bond with her chauffeur, Becky-san, a woman who dresses in male attire and drives with quiet competence. Nicknamed after the heroine of Thackeray's Vanity Fair, Becky-san is more than a driver; she is sharp-witted, well-read, and perceptive. Together, the two find themselves drawn into mysteries that unsettle their refined world.

The first narrative arc, Vanity Fair, plunges them into a bizarre and unexplained death. The key to the puzzle lies hidden in a novel by the celebrated mystery writer Edogawa Rampo. As Eiko and Becky-san piece together clues, their investigation uncovers layers of deception within high society. In later arcs, such as Ginza Haccho and Street Lights, they tackle a code that troubles Eiko's brother and a puzzling death during a film screening. Each case invites them to navigate the delicate social codes of prewar Tokyo while relying on Becky-san's extensive knowledge and Eiko's growing intuition. Their partnership deepens across the series, blending historical atmosphere with the quiet thrill of intellectual discovery.