Beszel and Ul Qoma are two city-states that inhabit the same space. The same street may have different names in the two cities, and the pedestrians and drivers in each city must avoid their counterparts in the other city while "unseeing" them. Citizens in one city must step over homeless people sleeping on the sidewalk or couples making love on the ground, again while "unseeing" them. The process of unseeing involves making distinctions between people based on the colors they wear, the way they carry themselves while walking, and other such subtleties. A person who crosses from one city to another without going through the proper channels (crossing through the official border in Copula Hall), who willfully sees what he/she should not be seeing--an offense known as Breach--will be immediately taken up into a third city that somehow exists between the two cities, also known as Breach.
Astoundingly, there may be yet another city somewhere in the interstitial spaces. The existence of this possibly mythical city, Orciny, is at the heart of the mystery that provides the narrative structure for The City and the City. A woman is found dead in Beszel; she is soon identified as a Canadian archeology student, part of a team excavating a site in Ul Qoma. She is also know to be a believer in Orciny. Inspector Tyador Borlu quickly realizes that she was killed in Ul Qoma, and, believing that Breach must have occurred, he petitions for the case to be taken over by Breach. A powerful government official makes sure that his petition is denied, however. Because the murder occurred in Ul Qoma, their police are tasked with solving the crime, and Inspector Borlu is sent across the border to assist. Borlu and his new partner, Qussim Dhatt, are soon investigating the disappearance of another student, interviewing unifs (supporters of unification) and nats (radical nationalists) and wondering what role Orciny plays in the case. Without saying enough to ruin the book for anyone who wants to read it, I will say that one of the characters ends up in Breach, so we get to learn more about this mysterious force and how it operates.
Mieville is known as a fantasy/science fiction writer, and The City and the City has fantastical elements. The "grosstopically" connected cities are beautifully imagined, providing a canvas on which Mieville can explore such themes (as identified by my son Kevin) as "tenuous cultural coexistences, being wilfully ignorant of one's neighbors, the weird arbitrariness of cultural differences." In creating The City and the City, however, he melded fantasy with the genre of noir crime fiction; interviews with Mieville suggest a variety of reasons for the choice, including a penchant for playing with different forms, a belief that the two genres are both types of dream fiction (he describes noir fiction as "dream fiction masquerading as a logic puzzle"), and a desire to write a book his Mum would have loved. Unfortunately for me, the mystery elements don't work as well as the fantasy elements (this may be because I've read a lot of crime fiction and not so much fantasy): the plot drags and the characters are flat.
I doubt that I will read any more Mieville (again quoting Kevin, who has read three other Mieville books and found them "dizzyingly imaginative, but so much so that he [Mieville] gets lost in the clouds"). Still, I'm glad I read this one, and I would recommend it to anyone who doesn't normally read fantasy novels but would like to dabble in a unique imagined world.
It may or may not have been Beszel, that we built, back then, while others may have been building Ul Qoma on the same bones. Perhaps there was one thing back then that later schismed on the ruins, or perhaps our ancestral Beszel had not yet met and standoffishly entwined with its neighbour. I am not a student of the Cleavage, but if I were I still would not know.
(Again in an interview, Mieville points out that Cleavage, the term used to describe the way the two cities came to be co-located, can mean either a splitting or a joining--and no one knows which applies in this case.)