Let me make it clear that of the thirty-three chapters in this novel only the final thirteen are set in the Troy of the Iliad.
In the opening twenty chapters, which I think are more engaging, Madeline Miller writes of Patroclus, an awkward young prince, who has been exiled to the kingdom of Phthia to be raised in the shadow of King Peleus and his golden son, Achilles. "The best of all the Greeks" - strong, beautiful, and the child of a goddess - Achilles is everything the shamed Patroclus is not. Yet despite their differences, the boys become steadfast companions. Their bond deepens as they grow into young men and become skilled in the arts of war (more so Achilles of course) and medicine - much to the displeasure and the fury of Achilles's mother, Thetis, a cruel sea goddess with a hatred of mortals. The developing homosexual relationship of Patroclus and Achilles is described sensitively and sensually.
In the last third of the novel Achilles goes down in my estimation and Patroclus rises. I liked this aspect of the story-telling. But on the whole I found the pre-Troy narrative more interesting.