Set in the generic far future city of -Not yet Named- in which humans have built towards the sky in the clichéd common-sense-defying extreme.
This divides the city into the LowCity which is above ground, but well below everything else (which makes it ‘below ground’ to the current inhabitants).
The MidCity where the can see the sky, but only as gaps between the buildings. Sort of like living in an old growth forest, only the forest is metal. *grin*
Then the HighCity or the SkyCity, which is just the folks who like in the topmost floors of the buildings.
Most of the PureStock (ie Humans that never left the planet) live in the LowCity out of habit. Most of the SkyCity folks are colonists that left and returned, or those that funded the exploration. Most of the money to be made is made offplanet at this point, but as habitable planets are a lot scarcer than had been expected (at least ones in the comfort range of the SkyCity-ians) most end up back on Earth.
MidCity are often those folks who work for the SkyCity crowd, although there are plenty of white color jobs in the LowCity as well. Everything depends on computers (or almost everything) and a lot of society is dedicated to keeping the interstellar networks up and running.
LowCity and MidCity aren’t slums, although sections of both are ‘not the best place to live’, much like current cities. Some folks swear by the LowCity because there is no solar exposure, and most people would rather live in MidCity than Sky, just because SkyCity folk are seen as obnoxious.