Alec Campion, the Mad Duke, is some twenty years older than in Swordspoint, but he isn't any less a trial to his family, friends, and enemies. Dividing his time between Tremontaine House and his Riverside house, the Duke Tremontaine hosts parties ranging from the risqué to the debauched, and lives a life of dissipation.
He also quietly makes political trouble for those intent schemes that would line their own pockets at the expense of the less powerful and the less well-connected. Aside from his own affinity for the dispossessed, it doesn't hurt at all that the principal plotter against the general good is his old enemy, Anthony Deverin, Lord Ferris. Into this political and social minefield, Tremontaine brings his niece, Katherine. With the stick of a revived lawsuit challenging his sister's marriage settlement and the carrot of permanently settling the lawsuit, he forces his sister Janine to send her daughter to him--with an absolute ban on family contact for six months.
Katherine arrives with happy dreams of fine dresses and a Season in town. She quickly learns that she will have only boy's clothes, and fencing lessons. Her uncle is having her trained to be his bodyguard.
As Katherine slowly learns her way around the duke's household, the city, and a sword, she also acquires a few friends, most notably Marcus, the duke's young assistant, and Lady Artemisia Fitz-Levi, a sweet but somewhat silly young lady of her own age, who nevertheless receives and accepts a proposal of marriage from the most eligible bachelor available--the widowed Lord Ferris.
Katherine's not happy to discover she'll be going to no respectable balls, wearing no dresses, and being received by practically nobody, but she does learn to enjoy swordplay and, with Marcus, trailing and investigating one of the Duke's visitors, whom she recognizes from her one very brief attempt to visit Artemisia. Unfortunately, the next place she meets Artemisia is at the Rogues' Ball. Katherine has come with the Duke; Artemisia with her fiancé, Lord Ferris. Lord Ferris, concerned that the flighty Artemisia might call off the betrothal that he's counting on for reasons of his own, has taken advantage of this evening away from Artemisia's family, friends, and chaperones to make sure she has no choice. Artemisia begs for Katherine's help, and Katherine's personal desire to avenge and protect her friend gets tangled up with the Duke's personal and political enmity for Ferris. Everyone's keeping secrets from everyone, and things start to spiral out of control.
Like Swordspoint, this is a really fine fantasy novel with not a hint of magic to be found in it anywhere.