The Stone Sky is the third book in the trilogy, so my discussion will include references to the previous two books, as well as commentary on the series as a whole. There may be spoilers.
The end is nearing, perhaps a permanent end. The moon is approaching, and it is up to Nassun and Essun to decide how the Plutonic Engine is going to be used, returning to Father Earth his long lost child—or ending the aeons-long war with a bang. Essun controls the key to the engine, the Onyx obelisk, but the system has an override at an island city called Corepoint in the middle of an ocean on the far side of the world. There the showdown between mother and daughter will take place.
The main narrative unfolds through Essun and Nassun’s twin story lines. First Essun. After her exertions in The Obelisk Gate, Essun fell into a coma and awakens to discover her world changed. Not only is Castrima on the move in a race to inhabit the now-depopulated city Rennanis, but now using orogeny causes her limbs to turn to stone—a process already underway. Essun simultaneously has to further her knowledge about how to use the engine and come to grips with her newly-imposed limitations. Almost despite herself, though, Essun has her friends, including her Stone Eater Hoa, the inventor Tonkee, and lorist, cum-conscript general, cum strongback laborer Danel. They will help Essun however they can.
As befits her childhood, Nassun’s story is more straight-forward. She is resolute in her determination to destroy the moon, carrying out the wish of Steel, “her” Stone Eater. Despite the dangers of a trip through the earth on a long-forgotten transportation system, Schaffa refuses to leave her side. This trip, which will take him near to Warrant (the Seasonal home of Guardians), and destroy him utterly.
Like its predecessor, The Stone Sky introduces a new plot thread that serves as the keystone in the overarching story being told by Hoa. Thousands of years in the past there were no stone eaters, no orogenes, and no seasons. In Syl Anagist life was sacred and the millions of people built the Plutonic Engine to harness the power of “Geoarcanity” by harnessing the power of life into an endlessly efficient system. This will herald in a perfect society that will reduce the number of people who need to be exploited to an absolute minimum. Of course any system requires some sacrifice. Each element of the Plutonic Engine (the obelisks) have to be infused with the life energies of the apostate Niess, a defeated people who were capable of manipulating these energies, and initiated by genetically modified, supposedly sterile people called Tuners capable of using both this magic and orogeny.
But just as Icarus flew too close to the sun, the people of Syl Anagist dug too deep and pushed too far. The consequences were dire, starting a war with Father Earth that threatens to destroy humanity.
Despite the central importance of the mother-daughter relationship over the course of The Broken Earth trilogy, Jemisin is at her best when tackling questions of exploitation. Some of these, such as Schaffa’s relationship with his charges, function as surrogate parent-child interactions, but more often they operate on system levels, such as the suspension of powerfully-orogenic children in stations to reduce seismic activity. Here, in addition to explaining the origins of the conflict, the entirety of the new plot arc asks the question whether it is possible to build a society without exploitation. The answer, obviously, is no.
The Stone Sky is a little bit slow in pulling the divergent threads together and at times substitutes answering questions about the world in place of plot development on the main plot. This is a function its structure, i.e. that it is Hoa explaining information to Essun’s final form, and the setup from the earlier books in the series that started with the characters apart. Thus in addition to the origins of the conflict, we learn about dead-civ ruins, the secrets of Guardians, and origins of Stone Eaters in this book. The Stone Sky is not merely an info-dump, all the same. First, the answers are all to questions raised earlier in the series and, second, the answers come from stories. All the same, the pacing of The Stone Sky is notable because it is fundamentally a race against time with the total apocalypse one of the potential outcomes.
For those people who delay reading a series until they know whether the author sticks the landing: Jemisin does. I would not be surprised if it does not complete the three-peat of Hugo awards, but it is nevertheless a deeply satisfying conclusion to a brilliant series.
Next up, I broke my streak of books written by women when I started reading Yasher Kemal’s modern folktale Memed, My Hawk.