The Industrial Revolution has escalated into all-out warfare. It has been eighteen years 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 Hetrodynes' return. At Transylvania Polygnostic University, a pretty, young student named Agatha Clay seems to have nothing but bad luck. Incapable of building anything that actually works, but dedicated to her studies, Agatha seems destined for a lackluster career as a minor lab assistant. But when the University is overthrown by the ruthless tyrant Baron Klaus Wulfenbach, Agatha finds herself a prisoner aboard his massive airship Castle Wulfenbach—and it begins to look like she might carry a spark of Mad Science after all.

Intrigue! Subterfuge! Circus Folk! In a time when the Industrial Revolution has escalated into all-out warfare, mad science rules the world... with mixed success. 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. Pursued by the ruthless Baron Klaus Wulfenbach, his handsome son Gil, and their minions (not to mention Othar Tryggvassen, Gentleman Adventurer), Agatha hides in plain sight among the circus folk, servicing their clanks and proving herself adept in performing the role of Lucrezia Mongfish, nemesis to - and later wife of - Barry Heterodyne. She also begins training under Zeetha, swordmistress and princess of the lost city of Skifander. Together, Agatha, Krosp, and the performers travel across the treacherous wasteland of war-torn Europa, towards Mechanicsburg, and the ancestral home of the Heterodynes - Castle Heterodyne. But with many perils standing in her way - including Wulfenbach's crack troops, mysterious Geisterdamen, savage Jagermonsters, and the fabled Storm King - it's going to take more than a spark of Mad Science for Agatha to get through.

Airships! Imposters! Mad science! Agatha Heterodyne is the last heir to the Maddest of Mad Scientist families, and now she's returning to her ancestral home, the warped little town of Mechanicsburg, to claim her inheritance. Unfortunately, it turns out that she's not the only one with that idea in mind. When an imposter claiming to be the rightful heir appears, Agatha must convince the artificial intelligence that animates her family's castle that she is, in fact, the new Heterodyne. But as Agatha soon discovers, this will be no simple task, as the Castle itself is completely insane.

In a time when the Industrial Revolution has become an all-out war, mad science rules the world—with mixed success. 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 all the revenge the mad Heterodynes of the past so richly earned.