There are two cities, Beszel and Ul Qoma. They are beside each other, and also overlap. The latter is something inhabitants of both lands, and any visitors to either, are supposed to ignore. See that person down the street, whose clothes and gait don't match everyone else? No you don't, or at least, you better not. Or else you're in breach and soon you'll vanish into Breach.
The specifics of how that circumstance came to be, Mieville doesn't really investigate. None of the characters in this book know, or if they do, they're aren't telling or even thinking it where any narrator can notice. It's just a part of their surroundings they accept and attempt to navigate with their investigation of a murdered archaeology student.
Tyador Borlu is the Beszel detective assigned the case, and he's a fairly typical fictional cop. Overloaded with cases and outwardly willing to find an excuse to dump this on someone else. Yet also dogged enough to keep digging even when it looks as though he'll get what he wants and the case will be declared breach and taken out of his hands. He annoys people in power by asking questions they don't like, he frustrates his colleagues and partners (and he ends up with those in both cities and more before it's over) by keeping his thoughts to himself and pursuing hunches alone, leaving them to deal with the fallout.
I didn't have any sense of the answer to whodunit, but I'm not sure if that was because Mieville did an excellent job masking it, or because it felt like the whole thing about the nature of the two cities, plus Breach, overshadowed it. Because everything about the investigation, every careful step Borlu has to take, is dictated by those circumstances. So the question of why things are like that, or how they got that way, feel important. But, you know, Mieville doesn't really answer that, not directly.
I did, however, see Borlu's fate coming from a fair distance off. Pretty much from the moment he was apprehended, I knew how things were going to end for him. I don't know if that's a good thing or not.
Reading this was a strange experience. I wasn't invested in the actual murder mystery, which seems bad, nor did Mieville answer the questions I did have, which seems unsatisfying. Yet I read through it quickly and easily, without any hesitation or question of whether I wanted to finish. So it must have done something right.
'The theft of the van and the dumping of the body in Beszel were illegal. The murder in Ul Qoma was horribly so. But what we had assumed was the particular transgressive connection between the events had never taken place. All passage had appeared scrupulously legal, effected through official channels, paperwork in place. Even if the permits were faked, the travel through the borders in Copula Hall made it a question of illegal entry, not of breach. That is a crime you might have in any country. There had been no breach.'
