The main characters are two children – a boy and a girl --that live in an underground city.
The people in the city are totally oppressed by the folks who run the place (I think they’re called “watchers”) but the city dwellers aren’t really aware of it. They don’t even know that their city is underground.
The girl is a “reader,” meaning that she knows how to read. This is unusual among the city dwellers, and in fact readers are persecuted by the other city dwellers, probably with the encouragement of the “watchers.”
Anyway, the children escape the city and end up in a city above their city that is populated by the “watchers.” They are soon found and expelled from the above-ground city.
I suppose I shouldn’t spoil the ending.
Thanks for your help!!