Beszel and Ul Qoma are sister cities, set cheek-by-jowl, but forever separated by a thin membrane that residents are forbidden to cross. This ethereal border is policed by a mysterious and powerful force called Breach that punishes anyone who crosses illegally. Inspector Tyador Borlú, of the Beszel Extreme Crime Squad, is called to the scene of a murder whose victim turns out to be a graduate student researching an ominous and mythical city named Orciny, which is said to exist within the membrane itself. Inspector Borlú must risk a dangerous foray into Ul Qoma to investigate the murder, and much to his astonishment, the possible existence of Orciny.