In Agatha H and the Siege of Mechanicsburg, Agatha Heterodyne, the last of the Heterodyne family, has returned to her family's hereditary town with the might of the Wulfenbach Empire hot on her heels. The only thing holding off an outright attack is Gilgamesh Wulfenbach, the heir to the Empire, who has joined Agatha and her friends inside the great artificially intelligent fortress of Castle Heterodyne. Now they must race to repair the mechanisms that once allowed the Castle to defend Mechanicsburg, before the rest of Europa shows up to take revenge....
With the help of Krosp, Emperor of All Cats, Agatha has escaped from the massive airship known as Castle Wulfenbach. After crashing their escape dirigible, Agatha and Krosp fall in with Master Payne’s Circus of Adventure, a traveling troupe of performers dedicated to staging Heterodyne shows - dramatizations of the exploits of Bill and Barry Heterodyne and their allies - who are unaware of Agatha’s connection to the Heterodyne line.
The Industrial Revolution has escalated into all-out warfare. Sixteen years have passed since the Heterodyne Boys, benevolent adventurers and inventors, disappeared under mysterious circumstances. Today, Europe is ruled by the Sparks, dynasties of mad scientists ruling over - and terrorizing - the hapless population with their bizarre inventions and unchecked power, while the downtrodden dream of the Heterodynes’ return. At Transylvania Polygnostic University, a pretty, young student named Agatha Clay seems to have nothing but bad luck.
In the third installment of the Girl Genius novels, Agatha H. and the Voice of the Castle begins as Agatha Heterodyne returns to her ancestral home, the warped little town of Mechanicsburg. There she must claim her inheritance by convincing the artificial intelligence that animates her family's castle that she is, in fact, the new Heterodyne. But this apparently simple task is made complicated in several ways: An imposter claiming to be the legitimate heir appears.