A murder mystery set in two cities that share the same geographic space. Beszel and Ul Qoma are two Balkan city states that just happen to be shuffled together like a deck of cards. This is not like East and West Berlin separated by a wall, or like Jerusalem with its various sectors. One house might be located in Beszel and the house next door in Ul Qoma but the residents in the houses have to act like the neighboring house does not exist and unsee their neighbors if they pass on the street. If a citizen of one city interacts with or even looks at a person, building, or object in the other city then they have Breached and can be disappeared by a secret police force called Breach that answers to neither city.
Like Asimov with his Three Laws of Robotics, Mieville sets up the rules governing these crosshatched cities and then begins finding ways to break the rules. The main character is a police inspector in Beszel who is tasked with solving a murder in which the victim was killed in one city and the body dumped in the other city, but no Breach occurred. The novel is a police procedural in which the only SF/Fantasy element is the setting, but the setting is fascinating and the novel would still be a strong political thriller even if all the speculative elements were removed.
While The City and the City does not conform to the space opera stereotype of science fiction, I would argue that it meets Darko Suvin's definition of science fiction as a text with the necessary and present condition of cognition and estrangement. The novel's unique setting is strange enough that the reader is forced to try and understand the world in which the characters exist in addition to trying to solve the mystery of the murder. Suvin is a good reference point for this novel because he is a Croatian scholar and Beszel and Ul Qoma are sort of like if Serbia and Croatia were physically the same country but the residents insisted they were different countries.