There is a major emergency at vamp HQ when an ancient vampire (probably the one of the first created, if vamp legends are correct) whom Leo has been holding captive, is helped to escape by traitors within Leo's clan. When Jane attempts to stop him, he blasts her with a massively powerful spell which puts her out of business for a while, and also leaves her with a semipermanent connection to him through her link to the evil artifact, the blood diamond.
The vamp leaves a trail of bodies across the city of New Orleans, including a massacre in a night club which riles up the local populace and police. To some extent, this plays into Jane's hands, as she feels responsible for a) not having killed him upon discovery, and failing to stop his escape, and b) failing to kill the traitorous vampiress who freed the monster when she had the chance a few books back.
The European vampires are still scheduled for a visit, and killing the ancient one, who has been merely presumed missing for a couple of centuries, would cause serious international consequences, if word got out. The witches are still scheduled for a conclave with the leaders of the Mithrans, as well, and these events could affect that rapprochement, too.
Jane must use all of her allies to defeat this ancient monster, including the leader of the witches in New Orleans, Lachish, and her friend Molly, the werewolf Brute, a whole host of Leo's scions, and her lover, Bruiser. The main things that happen here, from a plot arc standpoint, are that she finally embraces her nature as Warrior Woman, who must fight evil for a cause, while allowing the monsters a chance at redemption when possible, and finally accepting the fact that she's accumulated a bit of "family" in Eli and Alex Younger. She also manages to collect a couple of new magical artifacts along the way.