Maomao, a sharp-witted young woman trained in herbal medicine, spends her days quietly compounding remedies in the red-light district. Her life takes an abrupt turn when she is kidnapped and sold to the Imperial Palace, becoming a lowly servant in the vast complex that houses the Emperor's thousand concubines. Resolving to keep her head down until her two years of servitude end, her plans are foiled when she learns that the Emperor's newborn heirs have fallen critically ill. Her insatiable curiosity getting the better of her, she uses her apothecary knowledge to secretly investigate, discovering that the infants' sickness is actually a case of slow poisoning from toxic face powder.

Though she tries to remain anonymous, her actions attract the attention of Jinshi, an unnervingly beautiful and influential eunuch who oversees the rear palace administration. Rather than punishing her for meddling, he promotes her to the position of food taster for one of the Emperor's favored consorts, Lady Gyokuyou, forcing Maomao to use her expertise to solve a series of mysterious ailments, suspicious deaths, and palace intrigues.

The setting is a fictional imperial court inspired by dynastic China, where the rear palace functions as a gilded cage. Thousands of women live within its walls, their value determined solely by their ability to bear the Emperor a male heir. The series blends episodic medical mysteries with an unfolding serialized drama. Maomao investigates poisonings, arson, ghost stories, and strange illnesses, from a concubine allergic to certain foods to a room filling with lethal carbon monoxide. Her methods are grounded and scientific: she tests concoctions on the scars covering her own arm and deduces causes from subtle clues like scents and plant properties.

Throughout these investigations, Maomao develops a wary, often exasperated relationship with Jinshi, the only person who can match her intellect. While everyone else is charmed by his perfect appearance, Maomao remains stubbornly immune, frequently looking at him with disgust or boredom. Her lack of interest only fascinates him more, creating a slow-burn dynamic as he attempts to keep her in the palace while she dreams only of returning to her quiet apothecary life. Jinshi is supported by his loyal attendant Gaoshun, a stoic man who often acts as a minder for both of them. The story gradually reveals that Jinshi is not truly a eunuch, hiding his identity as the Emperor's brother by consuming suppressants, and that Maomao has her own complex family ties to the highest and lowest ranks of society.

Key narrative arcs include the mystery of the poisoned infants that first brings Maomao to prominence, the political maneuvering among the Four High Consorts of the Jade, Crystal, Diamond, and Garnet Pavilions, and Maomao's temporary departure from the palace that reveals the depth of Jinshi's attachment to her. Later episodes explore the Shi clan rebellion and the tragic backstory of Maomao's biological parents, including her father Lakan, a brilliant but face-blind military strategist, and her mother Fengxian, a former courtesan devastated by illness. The series balances dark themes of exploitation, infant death, and human trafficking with moments of dark comedy and intellectual triumph, driven by a protagonist who cares more about poisons than politics but cannot resist solving a puzzle.