"The Firebrand" of the title is Paris of Troy whose actions provide the impetus that unleashes a storm leading to the fall and conflagration of his city. His twin sister Cassandra - or Kassandra in this telling - is a prophetess who foresees the doom that her brother will bring but is unable to do anything to stop it.
(Note: The author renders Greek proper names that we are used to seeing spelled with a "C" to be "K" instead. Thus, Achaian is Akhaian, Achilles is Akhilles, Mycenae is Mykenae, and Cassandra is Kassandra. It's only a little disconcerting.)
Kassandra is the heroine here and it is through her eyes that we see Troy before, during, and after Paris steals Helen from Menelaus and brings her to the city to live. Kassandra is the daughter of Hecuba and Priam. Hecuba was an Amazon, raised to be a warrior. She gave that life up to be King Priam's wife and to bear him seventeen children, of whom Hector was the oldest son and Kassandra the oldest daughter. (Hmmm...why isn't it Hektor and Hekuba, I wonder?)
Kassandra proves to be the rebel in the family. She is blessed with foresight and cursed with the fact that no one will believe her prophecies. She becomes such a nuisance in the palace that her parents send her away to the Amazons to be fostered for a few years by their leader, Hecuba's sister. Living the life of an Amazon warrior, Kassandra later remembers, was the happiest time of her life. While with them, Kassandra finds her calling as a priestess. Upon returning to Troy, she determines to dedicate herself as a virgin priestess of Apollo.
The book carries us through the long years of the siege and, again, we see it all through the women's eyes, a unique perspective as opposed to the usual masculine blood and sandals tale of Troy. The men, as seen through these women's eyes, are little more than rambunctious boys playing at a game called "War." They love it and live for it and are unspeakably bored when there is a lull in the fighting.
Over the years, Kassandra travels more than an ordinary woman of Troy ever would have. In her guise as priestess, she goes to Colchis to seek wisdom of serpents. She travels through the lands of the Amazons and the Centaurs - I mean, Kentaurs - and meets some of the very last of their kind before they are exterminated. She finds a baby girl who has been exposed and decides to adopt and raise her. She has, in fact, many adventures before she returns to Troy to await her fate.
There, she takes as a lover Aeneas the husband of one of her sisters and she foresees that he will survive the sacking of the city and will found a new city. She will survive, too, as the concubine of Agamemnon. The last of the book tells us what happens to Kassandra after Agamemnon takes her back to Mykenae where he meets his doom at the hands of Queen Klytemnestra, the mother of his daughter Iphigenia whom he had sacrificed to the gods. All of this is more than Homer ever told us, but then it was a "woman's tale."
Marion Bradley Zimmer blended together the myths of the gods and the Homeric feats of heroes, stirred with a bit of archaeological fact and legend and she came up with a very interesting brew and a whole new perspective of the story of Troy. It kept me interested all the way to the end.