What we have here is a fictionalised version of a set of possible events leading up to the (SPOILER!) big eruption of Vesuvius in AD 79.
I’m no historian, but I gather by the long list of references and acknowledgements at the end of the book, I assume that the narrative of the geological events is reasonably accurate! Of course, the interactions of the characters are an interpretation and elaboration.
The motivation of the central character Attilius, is firstly the investigation into the failure of the water supply to the surrounding town and area from the aqueduct Aqua Augusta. Later we move into a typical “lets rescue the princess” (well, she’s not a princess.. but you get the general idea) plot line.
Of course we know that come volcano day, things are going to go badly wrong. The descriptions of the event itself are limited to the terminology that would have been available during that time period, so you can understand the frustrations of the people who are attempting to comprehend something for which they have no frame of reference.