In the spring of 1889, New Orleans spinsters Fanny Newcomb, Sylvia Giddings, and her sister Olive establish the Wisdom Hall Social Settlement. Located in the city's Irish Channel, Wisdom Hall is dedicated to improving the lives of Irish and German immigrant women. The three cohorts—self-proclaimed "Wisdom Women"—recruit Irish Channel residents to attend their classes in English, typewriting, and ledger-keeping.

As the daughter of a lawyer, Fanny's life has been shaped by her father's quest for fairness and her own keen intelligence. Fanny is dedicated to justice for all, but she is not above twisting the truth—or even stealing—if it helps her achieve her goals.

Sylvia, a charming, cunning, and elegant socialite, yearns to educate Irish Channel women and help them escape their predictably-restricted futures. Her younger sister, Olive, is a recent medical school graduate, and operates an infirmary at the rear of Wisdom Hall.

As A Certain Justice begins—Olive discovers the corpse of a young woman who has been strangled and left to die just blocks away from Wisdom Hall. The next day's newspapers proclaim that the woman was a prostitute, viciously slashed and butchered—just like the recent victims of London's Jack the Ripper. Fanny quickly realizes that the dead girl is her favorite student, Nora Keegan, a recent Irish immigrant. Though Fanny is shocked to learn that her student worked as a prostitute, she vows to discover Nora's murderer while attending her funeral at Metarie Cemetery.

Rumors swirl about New Orleans that Jack the Ripper has landed on American shores. Hysteria grips the Crescent City. To placate the outraged Irish community, the police arrest German immigrant Karl Schultz, Sylvia's carpenter. Appalled by the police's political maneuvering and confident about Karl's innocence, Sylvia enthusiastically joins Fanny in her search for Nora's murderer. Olive—who knows from the medical evidence that Nora was not killed by a Ripper's knife—pledges her support to the investigation.

Suspects abound, including Olive's surgical mentor, a Yankee ambulance doctor, a shabby Canal Street photographer, and even a priest and his seminarian brother.

The Wisdom Women are assisted by Cousin Ede, a socially prominent and savvy New Orleans native who operates a Decorative Arts School in the French Quarter. Cousin Ede knows everyone in New Orleans—and most of their secrets.

But the police are scrutinizing the Wisdom Women's activities. Irish Channel-born Detective Daniel Crenshaw attempts to outguess the women, although he is especially challenged by Fanny's passionate intellect and sound intuition.

As the Wisdom Women pursue their murder suspects, they are plunged into the worlds of prostitution and pornography, of law enforcement and medicine. The search for justice takes the women from the waterfront of the Irish Channel, along the Mississippi docks, through the eroding splendor of the French Quarter, into the elegant Hotel St. Charles, and among the girls who work in the houses of prostitution on Rampart Street.

Throughout their investigation, the Wisdom Women face down dangers threatening their lives and livelihood. In the end, they discover A Certain Justice not only for Nora Keegan, but for themselves as well.