The world is divided into city states protected by giant skyborne sunshades, and the surrounding skylands, inhabited by the skykin who are bonded with a symbiote which changes them so they can resist the unfiltered sun's radiation. This picks up the story begun in Hidden Sun. It's two years later. In Shen, Rhia is still hidden away in her garret in the Harlyn town house, making her observations, corresponding with other natural scientists in other shadowlands, and working on her own theory that their world travels around the sun, not the other way round. Etyan, her younger brother and the heir to the Harlyn lands and fortune still refuses to own up to his responsibilities. Since he was changed by a (mad) scientist in the first book, he's halfway to being skykin himself, which is just as well because his girlfriend/lover, Dej, is all skykin, though estranged from them. Etyan and Dej are keeping out of the way of polite society in a self-built shack in the Umbral forest so they can come and go to the Skylands as they please. Etyan has been cleared of his great crime in a court that was bound to find a noble 'innocent', but the deed still haunts him. And when Dej discovers his secret her reaction is devastating.
When Rhia is accused of heresy for her scientific ideas she stands in danger not only of losing her life's work, but her life itself, if the court finds her guilty. Being the cousin of the ruling Duke can't save her. But it turns out that the night sky holds something far more devastating than losing her work.