Quitting her husband's house and moving back in with her horrible family, Lady Maccon becomes the scandal of the London season.
Queen Victoria dismisses her from the Shadow Council, and the only person who can explain anything, Lord Akeldama, unexpectedly leaves town. To top it all off, Alexia is attacked by homicidal mechanical ladybugs, indicating, as only ladybugs can, the fact that all of London's vampires are now very much interested in seeing Alexia quite thoroughly dead.
While Lord Maccon elects to get progressively more inebriated and Professor Lyall desperately tries to hold the Woolsey werewolf pack together, Alexia flees England for Italy in search of the mysterious Templars. Only they know enough about the preternatural to explain her increasingly inconvenient condition, but they may be worse than the vampires -- and they're armed with pesto.
I strongly encourage you to read the books in order. They'll make much more sense that way.