The book is set in a city comprised entirely of towers of living bone which rise high above the cloud-line, which people travel between using hang-glider-like wings. The protagonist, Kirit Densira, aspires to be a trader like her mother, visiting far-off places and carrying vital supplies, but first has to pass her tests. This is complicated when she ends up on the wrong side of the rules, bringing herself to the attention of the city's secretive protectors, the Singers. Kirit has a rare talent they're interested in, and they're not inclined to let her go. As she gets more and more entangled in the politics of the city and the Singers, she finds out that a lot of things aren't working the way they ought to, and the city's people aren't actually as protected as the Singers would have them believe.
The book contains examples of:
- The Call Knows Where You Live: Kirit doesn't want anything to do with the Singers, and doesn't care that her special talent would be useful to them (and, they say, to the city). However, they make it clear that they'll ruin all her alternatives until she "chooses" to join them.
- Evil Former Friend: From Nat's perspective, Kirit herself is this, due to seemingly aligning with the Singers despite knowing what they did to Nat's father. However, they eventually accept that they were actually seeking the same basic thing by different routes, and reconcile.
- He Knows Too Much: This is why the Singers executed Nat's father; he knew that Singers possessed knowledge of how to safely navigate at night, which would be very useful for non-Singer fliers. Even if the Singers had been prepared to share that particular technique, people would wonder what else the Singers were hiding... and would now be better able to discover just that, since night would no longer cover Singer activities which people would get really angry about.
- How the Mighty Have Fallen: It turns out that Tobiat, a scruffy, mad-seeming vagrant in Kirit's home tower, was once one of the exalted Singers.
- Invisible Monsters: A major threat to the citizens of the city are skymouths, flying creatures which are completely invisible except for the mouth they open to eat you with. Naturally, they're hard to defend against, and Kirit's ability to repel them is why the Singers are so keen to force her join up.
- Monster Organ Trafficking: Skymouths are highly dangerous, but also provide useful materials; in particular, sinew which allows the construction of bridges between the towers which constitute the city. Singers, who are responsible for protecting the city, maintain some of their clout thanks to the prosperity this sinew brings them. It's eventually revealed that the Singers are secretly breeding a horde of captive skymouths rather than killing them in the course of protecting the city, as they claim.
- Monster Protection Racket: Kirit is told that she the ability to repel skymouths, a major threat to the safety of the city's inhabitants. This power is real, but the people recruiting her don't want her to repel skymouths so much as help herd them; the Singers breed skymouths and control their supposedly random migrations, while taking credit for protecting the city.
- Mutant Draft Board: There isn't any rule saying that people with the ability to repel skymouths have to join the Singers, but Wik makes it clear that alternative plans Kirit might make will be thwarted. This includes declaring her to have failed vital tests based on a technicality that isn't applied to anyone else. She still doesn't accept; rather, she tries to challenge the Singers directly, and even when that results in capture, she still treats joining as a reluctant bargain rather than an obligation.
- Nemean Skinning: The invisibility of skymouths is inherent to their skins, a fact which comes in handy when Kirit wants to make herself hard to find. It's not a very clean process, but it works.
- The Order: The city doesn't really have a centralised government, but Singers are the protectors of the city and keepers of its Laws. They keep apart from ordinary citizens, and are secretive about their techniques and operations. They're highly identifiable, having silver tattoos, and are both respected and feared. At the end of the book, it's revealed to the world that they've been lying about the protection they provide; to reform the Order, there are going to have to be major changes, including ending its separation from the people it's supposed to serve.
- Refusal of the Call: Kirit isn't interested in using her gift by joining the Singers; she'd much rather be a trader like her mother, which would have averted the whole plot. However, the Singers are quite prepared to ruin her chances of having that life.
- Screw the Rules, I'm Doing What's Right!: Kirit decides to expose the Singers' dangerous duplicity about the skymouths by winning a Trial by Combat, after which her opponents could no longer deny her permission to speak. Then she realises... why is she bothering to play by the rules of the people she's trying to expose? Why wait for their permission? Instead, she shouts out the truth during the fight, letting the whole audience know it regardless of whether she wins or loses.
- Skyscraper City: A fantasy version; the city consists of a cluster of living towers made of bone, high above the clouds and slowly rising. Some are connected with bridges, but the fastest way to travel is by strapping on wings.
- Star Scraper: The bone towers which constitute the city all reach well above the clouds, and their lower levels are essentially abandoned; as far as their inhabitants are concerned, the above-cloud portions of the towers are the whole world.
- Those Magnificent Flying Machines: Use of hang-glider-esque wings to fly around and between the towers of the city is routine, and Kirit is very keen to pass her tests and become an airborne trader like her mother. These wings are also used for combat, both against monsters and in duels (including Trial by Combat). There do not seem to be any larger air vehicles, however; or powered ones.
- Trial by Combat: It's legally permissible to challenge the Singers, who are responsible for interpreting and enforcing law, in single combat. This can be done both by ordinary citizens and by Singers who disagree with their leaders' decisions. Combat takes the form of airborne duels using the hang-gliders-like wings commonly used for transport.
- We Used to Be Friends: Kirit and Nat, her friend from her home tower, eventually come to odds due to one of them challenging the Singers and one of them now being a Singer. Their Trial by Combat ends with Nat apparently dying, but actually surviving. They later reconcile. Another example might be Kirit and Sellis, a fellow Singer trainee, but that friendship is more illusionary based on not ever really understanding where Kirit was coming from.
- Your Eyes Can Deceive You: As part of her training, Kirit is required to move around blindfolded. This is to encourage her to use other senses, which are necessary if she is to learn to fly at night.
