This novel is set in Ancient Rome, at the time where the Republic was in decline. Gaius Julius Caesar was still a nobody - a very ambitious one but not yet considered a serious player.
The protagonist is a very junior member of the powerful Caecilius Metellus family. He has a government job of minor importance which involves dealing with crimes. As we learn from the book, murders were not considered such a big deal back then (arson was considered far more heinous), and murderers were rarely discovered and prosecuted. The protagonist Decius, though, turns out to have a unique talent at (as well as interest in) just that. For example, by a flash of inspiration, he lets a physician examine a dead body, hoping that he might be able to tell something about the way the victim was murdered - an idea unheard of in Rome until then.
At first, his investigation is just a routine. Soon, though, the protagonist uncovers something very big and nasty - in fact, something of national importance and larger than he can handle.
What makes this book an extraordinary delight to read, are the extremely life-like, credible characters. It is also interesting to see methods of criminal investigation that are completely different from what we are used to. Rome had a fascinating political system and an almost-rule of law at the time most of the humanity had just recently descended from the trees.
Unfortunately, the plot could be much better. The protagonist keeps acting very irrationally. He learns that something very bad is going to happen, but he succeeds in thwarting it by sending a message to a person in a key position. Then he could let the matter rest, but he keeps pushing it, insisting on bringing the guilty parties to justice, even though it should be obvious that they are much too powerful for him. Especially, the way he keeps bombastically talking about "saving Rome" when it is quite obvious that Rome is not in danger, is ridiculous. Also, I am allergic to male protagonists who are women's doormats. Maybe the most disturbing thing, though, was that the protagonist chose to be disloyal to his patron for a rather trivial reason.
My minor complaints concern the spelling of names. Quite annoyingly, the author has chosen to write "Pompey" instead of "Pompeius". I can well understand that the English-speaking world is used to the spelling "Pompey", but he uses the correct Latin spelling for all other names (like, he writes "Marcus Antonius", not "Marc Anthony"). What is the point of choosing one from all Roman names and write it in a non-Latin spelling? Especially when he uses the Latin spelling for the man's full name: "Gnaeus Pompeius Magnus", not "Gnaeus Pompey Magnus".
Also, the author is inconsistent in his use of G and C.
When I read this book the first time, I felt it was one of the best books I had ever read in my life. By now that I have read all the novels in the series, this one is barely 5 stars.