In a new departure for fantasy that will challenge the head as well as the senses, the body of a mutilated foreign student is discovered in the streets of the crumbling European city of Besźel. Cases like this are run of the mill for Inspector Tyador Borlú of the Extreme Crime Squad - until his investigations uncover evidence that the dead girl had been involved in the political turmoil between Besźel and its "twin city" of Ul Qoma, in a universe of parallel worlds. For Borlú to solve the case he will be required to go on an odyssey of the mind, a journey across borders that must be "unseen", to the city which is both a complement and rival to his own.