Wow, this book was utterly original, I can't imagine how the author even came up with the idea for his setting. It appears to be set in modern day earth, or perhaps slightly in the future - Coca Cola and Tom Hanks movies are mentioned - but in an alternate version in which the neighboring countries (city-states?) of Beszel and Ul Qoma exist. I didn't catch whether it's ever specifically stated where in the world these exist, but the names sound somewhat slavic to my completely ignorant ear so if I had to guess I'd guess eastern Europe/former USSR state type area. Anyway, these two countries both claim the same area of land ... and rather than fight over it or divide it in half, they BOTH live there. Yes, both countries physically occupy the exact same space.

When I initially read the book blurb I had somehow got the impression that one city was in the "real world" and the other was in some kind of mystical parallel dimension that it was occasionally possible to cross into. But nope, both cities, in two different countries, share the same streets, borders, even parks and structures. The author is taking the human tendency to ignore what we don't want to see to a ridiculous extreme. The land in the dual city where the story takes place is designated as belonging to Beszel, to Ul Qoma, or "cross-hatch". In cross-hatch areas, citizens and traffic of both countries can travel, but have to "un-see" each other. They have adopted traditions of dress, habits of movement, even expression, so that citizens of either find it easy to immediately tell who "belongs" and who is in the other city and is therefore not actually there, and must be ignored. Beszel citizens can only enter Beszel or crosshatch streets, parks, shops, and so on. Ul Qoma citizens likewise. If you live in Beszel, to visit someone in Ul Qoma who may literally live physically next door to you, you would have to go to the official border between the two, pass through customs, and then walk back to the physical location you started in, but you'd now be in a different city (and now could not enter your own house without re-crossing the border).

The existence of these two entirely separate cities is a shared group construct that citizens of both maintain. However, a secretive force known as Breach is responsible for maintaining that citizen, and if anybody does try to break that convention, Breach will immediately step in and vanish them away to be dealt with. This could be as innocent as even looking too long at someone from the other city, let alone speaking to or touching them.

The novel opens with Inspector Tyador Borlú of the Extreme Crime Squad in Beszel. As he investigates the murder of a young woman found in Beszel but who appears to have been murdered in Ul Qoma, he initially assumes it is a matter for Breach to deal with and refers the case to them. However, when it turns out that the murder has in fact been very carefully planned to avoid invoking Breach, the case ends up in the hands of himself and detective Qussim Dhatt of Ul Qoma. As they try to figure out who would kill this woman and why, not to mention how, they uncover bigger questions about what else may be hiding between the two cities, unseen by either as both sets of citizens un-see people and actions they assume are "other".

It took me a little while to wrap my head around the whole concept enough to get into the story, but once I did it was really interesting and such an incredibly unique setting as well. If you like your brain to get a little exercise imagining completely foreign concepts, you should definitely pick up this book! It's also a pretty good detective story also.