Hundreds of years before, the Devouts, then known as the anti-machinists, toppled the world's power structures and machines, and rushed in to fill the gap with their brutal rule.
The Devouts, and specifically Brother Poosk, are down but not out of this fight for control of the world. Brother Thrawn is out of the picture, thanks to Petrel, from the icbreaker ship Oyster, Rain, from the submarine called the Claw, and the robotic boy Captain, who doesn't feature as much in this book, although he's there at the pivotal end, and Missus Slink and Mister Smoke, robotic rats.
But the scientists didn't go down without a fight. They've laid the kernels for a scientific comeback -- not just on the Oyster, but also in a group of people called the Fetchers, who hold a key to rejuvenating science for all. Gwin is a Fetcher, a land-bound family of bards, who travel and guard the songs handed down from the scientists. But with her mother gone, her brother blinded and her father taken captive by Brother Poosk, she's not sure how she can help -- until just the right moment.
Each girl -- Petrel, Rain and Gwin -- play their pivotal parts in bringing light back to a dark age, and saving the Captain, and, if not defeating the Devouts, carving out a sliver of hope for the coming age.