Inspector Tyador Borlú of the Extreme Crimes Squad in Beszel is called to the scene of a murder; a young woman is discovered dead. Beszel is one of a pair of city-states, the other being Ul Qoma, which exist adjacent to each other such that they even share streets. The border cuts through parks and runs along pavements. But it is is a serious crime for the citizen of either city to even acknowledge the existence of the other. They bring in a mental blur filter and immediately look away.
All residents can identify the people, buildings and vehicles of the other city, and nearly involuntarily forget that they even observed it. Why? Because to do so is a crime called breach, policed by an authority higher than any single person or body, called “The Breach” who are a law onto themselves. People who commit breach are “disappeared”. No questions asked.
Anyway, the woman is revealed to be a Mahalia Geary, a Canadian student at the university in Ul Qoma. Borlú is all set to wash his hands off the case since it involves breach, but receives evidence that the murder vehicle crossed the border legally, with a permit, and “The Breach” would not be involved. Now Borlú has to work with the police across the border in Ul Qoma to unravel the case, which progressively gets murkier.
The world building in the book is beyond anything I have seen, and despite the complexity of the idea, Mieville never drops the ball, keeping the narrative tight and even and the plot moving steadily along. An absolute delight to read.