Humanity are sailing through the stars, having left the Earth in a vast fleet of ships in order to find a new home to inhabit. They've been sailing for 300 years, enough time for a stratified social structure to develop based on ship positions, with the wealthy and privileged at the front, and the poor and desperate at the rear.
Aisha is from the very bottom, the seventh tier, and is struggling to keep her family alive after the death of her parents. Her sister is relatively healthy, but her brother has caught the "wasting disease", an illness that runs throughout the ships. Medical care costs money, and Aisha has only one possible source. She must become a Scela.
The Scela are cyborgs, with each of them being upgraded from a basic human form to act as enforcers for the leading council, the "General Body". These man-machine hybrids are sliced open and surgically enhanced, with a robotic spine and nervous system being the core of an artificial intelligence onto which other metallic parts can be fitted. That robot mind now shares her brain, acting to temper her impulses or drive her to violence if they align with the General Body's will.
It also ties her into the exosystem, a shared mind which she occupies with Key, Pravaa and Woojin, the others who became Scela when she did. It passes their thoughts between them, allowing them to act more in sync as a team, but also giving away all of their secrets. It can also override their control of the Scela systems, driving them to perform brutal acts that they wouldn't otherwise, under the control of the General Body.
Key is the other viewpoint character, and is something of a mystery, having no memory of signing up for the Scela procedure, and having large gaps in her memory in general. She's from the front of the fleet, and has little time for the squabbles of the lower classes, or the brewing rebellion of the Fractionists, who want to split the fleet and hunt for planets in more than one direction.
It's a pretty good sci-fi thriller, with a relatively common class struggle and revolution plot, but makes up for it with the interesting mind-body duality bits about the way the Scela impacts on their lives. Been a while since I've seen a good shared mind idea (looking at you, Dollhouse) and this was done rather well.
Plot's a bit thin on the ground, but it's got some nice action sequences of a decent technical intricacy, and a good bit of tension as the ending ramps up, with some strong horror elements around the artificial mind control. Fun read, and cracked through it quickly.