Syd Craft is a brilliant detective known for solving the most difficult cases, yet he harbors a secret: he hates the blood and horror of his job and would much rather spend his time reading romance novels. Born to detective parents and pushed into the career by a series of childhood traumas, he maintains a cool, professional demeanor in public while privately seeking escape in the works of his favorite author, Levie Oleyn. The central irony of his life is that despite his genius-level observational skills, he is completely oblivious to the romantic intentions of the women around him, and the women who attract his interest often turn out to be the criminals he is hunting.

The story is set in a city resembling 19th century London, where Syd operates his detective agency. His young assistant is Elio Evelyn, a cheerful and popular teenager whom Syd hired expecting a male assistant because talking to women makes him nervous. Unknown to Syd, Elio is actually a count's daughter who dresses as a boy. More significantly, Elio is secretly Levie Oleyn, the romance novelist Syd idolizes. When overwhelmed by her feelings for Syd, she pours her confessions into notes that are collected and published as her novels, never suspecting that Syd is her most devoted reader.

Police Inspector Souffle Flamberry reluctantly relies on Syd's help to solve the toughest cases, considering him her fiercest rival and dreaming of one day surpassing him. Both are unaware that they were childhood friends who promised to marry, separated after a robber murdered Souffles parents. Syd is able to speak with her because her forward manner slightly intimidates him, though he does not recognize her as his first love.

Lulu Chatnoir, another fan of Levie Oleyn's novels, has a secret identity as Phantom Thief Mistcat, the seventh generation of a cat burglar family that considers the Crafts mortal enemies. Unlike Syd, Lulu immediately perceives that Elio is a woman. She begins visiting the agency more frequently, which Syd misinterprets as romantic interest in his assistant rather than in himself. Her attempts to seduce Syd for tactical advantage repeatedly result in awkward situations.

The narrative unfolds through a series of cases that Syd solves while the women in his life maintain their hidden identities. Hakua Metalica, the brilliant but lazy daughter of Syds landlord, also orbits this circle as a sister-like figure. An antagonist named Zero Craft, a mysterious relative who claims to love Syd, orchestrates various crimes including the Elfy bombing incident, promising to keep sending difficult cases his way.

As the series progresses, the romantic dynamics intensify. During a case involving a bomb threat on a luxury cruise ship, Syd and Souffle go undercover as newlyweds, forcing Elio to witness their feigned intimacy from a hidden position. The harem dynamics become the central focus, with each woman hiding her true self while vying for Syds oblivious heart. In its final chapters, the series concludes with Syd kissing Lulu and Elio, but when the time comes to choose, a sudden crime allows him to postpone the decision, leaving the three women to continue their rivalry indefinitely.