Imagine two cities, one right on top of the other, but entirely segregated. That's the premise of Miéville's book, but unlike his own Un Lun Dun or Gaiman's Neverwhere, there is no magic at all. Instead the cities and their peoples are kept separate through rigidly enforced laws. Besźel and Ul Qoma have different streets, buildings, architectural styles, histories, and businesses; the citizens have different languages and scripts, wear different colors and fashions, even move differently. And hovering above the two is the shadowy Breach, which moves swiftly to capture any one who, well, breaches by somehow intruding into the wrong city.
There are obvious parallels with Un Lun Dun, but unlike that young adult book, this one is noirish in its tone. Miéville convincingly portrays a completely different culture, even changing the grammar and vocabulary of his writing to give the impression of a narrator who speaks a different language and has, by necessity, an alien way of viewing his environment. Miéville also takes what could be a silly premise in the hands of another and shows how the weight of the rule of law, and the natural human tendency to prefer order over chaos and to respect authority (and, of course, to fear punishment), could lead to a situation where people conscientiously and consistently refuse to see what is literally right in front of them because it is part of somewhere else. It culminates in an incredible scene towards the end of the book where the authorities of both cities are unable to arrest a man, or even look at him, because they cannot tell which city he is in. "Schrödinger's pedestrian," indeed!
It's no coincidence, of course, that Besźel and Ul Qoma are located in eastern Europe. The real-world tensions that lead to balkanization are literalized here, and like Neverwhere Miéville's book is ultimately about the way we cling to a worldview of Us v. Them, the way we become adept at ignoring what is different from us or what makes us uncomfortable or what we just don't want to think about.