Agatha adventures! Spoilers for earlier books ahead!
Indeed, the opening scene suffered a bit from being in the middle of a fight that ended the previous book's cliffhanger.
So first she has to deal with the rampaging train, and some unexpected arrivals -- and some cake -- before she goes on the City of Lightning that is Paris on her quest to free Mechanicsburg.
Involves Zeetha's heroically facing her fears and eating cake, a lot of advertising, an uncle of Violetta's, an edged conversation after Agatha's locket does not keep in Lucrezia once, an arranged marriage, an informative puppet show, selling information about the weasel, and more.