This book, as many have noted, was an improvement on the previous book, mostly because it showed such a depth of character development as opposed to the previous ones in the series.
In this book, Flavia and her friends are still around Pompeii when they meet the incredibly charismatic Felix Publius. Other people have noted the main plot points of the book, so mostly I'll concentrate on what I thought were the noteworthy character developments.
Essentially what happens is that the young Flavia meets Felix Publius, as do her three friends, and the man is a very influential, wealthy, and highly charismatic man, and each are affected in turn. There is great depth of detail as to how he is able to work his charisma in different ways with each of them. For young fearless Lupus who has quite a sad backstory, he lets him hold the reins of the horse cart and even inducts him into his meetings with his "soldiers." For Flavia the bookworm, he tests and acknowledges her learning and presents her with a priceless gift of a painted cup in careless disregard of its monetary value. His charisma is very well-done and one of the best features of this book, as I think it does serve to let young readers understand this aspect of dangerous people. Only Nubia does not like him and thinks he is splitting apart the friends. Meanwhile, Nubia is befriended by an African ex-slave and wooed into wondering whether she should run away or not.
Felix also has three daughters, the oldest of which is a massively spoiled and carelessly cruel beautiful girl who beats her slave-girl with regularity and locks her in a chest for misdoings. Her actions towards her slave and Flavia's growing infatuation with Felix leads Flavia to start mistreating Nubia, which in turn causes her to consider seriously about running away. At the same time, Polla, the beautiful daughter, is enamored of Jonathan and treats him with fawning adoration while she ignores the rest of the group.
It is a situation ripe with different character dynamics and very different from the earlier books and even the later books, many of which are taken over with Jonathan's pessimism. It is also an interesting take on Flavia's infatuation with a much older charismatic man, who may or may not be the villain. This infatuation has a later follow-up in the book, "The Sirens of Surrentum," which was a book with the same emotional vein as this one, with Flavia being a strong protagonist instead of fading out to Nubia's introspection.
The only quibble I would have with this book would be Nubia's inner voice which condemns one character and praises another, both judgments which are proven wrong by the end of the book. This is strange because by the end of the series, Nubia has developed a superpower of her own, being that she can communicate with animals and instantly suss out the good and bad guys. It seemed like it was a red herring in this book and thus was a strange setup for her later "prophecies." It also doesn't seem in line with where the book started, with the strangely charismatic Felix to be completely harmless, as the initial message seemed to warn children away from such people.