In the city of Philadelphia, in the 1840's, there was great unrest in the city. So much so that Lippard's novel became the best-selling novel until Uncle Tom's Cabin, another moralistic criticism of the social inequality of America. The Quaker City is built around real legal cases, but the cases in the novel are only related. In the novel, we see two men on trial for murder, one wealthy, and one not. The wealthy man is a powerful figure, on trial for poisoning his wife after learning she had an affair. The other man is on trial for revenge killing his sister's rapist.
The story centers around an underground brothel where powerful men abuse innocent victims and enjoy unspeakable debauchery. The patrons of the Monk's Hall establishment are generally upper class, notable citizens. Ironically, the Quaker influence implied by the title is nearly absent, because that's likely how it felt in Philadelphia, a city known for his moralistic, others-focused ethics, where that religious influence was nowhere to be seen.
Ultimately, through the conniving of an unethical lawyer, the wealthy man is let off, and the more just man, the man who killed a violent rapist, is prosecuted with the full extent of the law.
Finally, a Quaker appears in the final moments of the drama, and he makes a few statements before leaving Philadelphia on a boat. This might represent the absolute injustice of Antebellum New England in Lippard's view, so defiled by its vices that even the founders cannot withstand the evil, and they flee. Philadelphia is left without its moral backbone.