Agatha gets on a train! Spoilers ahead for the prior books. Though it does begin with a sharper delineation of before and after than the ones just before it -- on the other hand, the tale clearly continues after.
Amusing contretemps while everyone in her party gathers, starting with the way she has to confess before being allowed on -- tell truly who she is and where she is going and why. The journey does get interrupted by a change in route and a necessary stop. Adventure and comedy along the way. Meanwhile Martellus and Gil are looking for her -- with Gil doing a sideline in trying to work out how to save those in Mechanicsburg from the effects of his father's time-stopping device. While birds age so rapidly they not only die but turn to dust he does rescue someone.
An intruder begging for protection discovers it does matter whom she asks, bears smelling something make a discovery, a cook organizes a defense, a Spark gloats about how other Sparks invite him into their territory to make the trains run, Wooster explains something to Agatha who thinks it might be rather romantic, Bang gives Gil good advice, an avalanche occurs, Martellus does something that actually makes him look good in the readers' eyes, Violetta holds forth on her family, and much more.