Borlú opens his eyes. He breached, and now he's under Breach control. If Mahalia and Katrynia discovered that there is a third city, that would leave the cities at war. Breach want to know about Orciny. They want Borlú to find the true link between Mahalia, Bowden and Borlú's subversive wife.

Borlú is interrogated by the Ul Qoman Militsya and is promptly taken to the Militsya HQ. He broke the terms of his agreement with Dhatt and she is furious – his possible deportation is out of her control. She's got him under constant surveillance. Dhatt knows this case means more to Borlú than being just another murder.

Borlú encounters another roadblock with his investigation into Mahalia's death when Commissar Gadlem tells him that the case has been taken over by the Ul Qoman Militsya. Gadlem allows Borlú to stay on the case and travel to Ul Qoma, but purely on an advisory capacity: the Ul Qoman Militsya are now leading the investigation.

When the body of a young woman is found in the city of Besźel, Inspector Tyador Borlú of the Extreme Crime Squad is called to investigate the murder alongside Officer Lizbyet Corwi. Although Detective Naustin presumes her murder can be easily explained, Borlú notices strange similarities to an old case.
The young woman's body was found at the Bulkyá Docks – an area known for its close overlap with the city of Ul Qoma. Although Ul Qoma and Besźel occupy the same geographical space, the cities have a dangerous and volatile relationship with each other, and citizens are trained from childhood to unsee everything in the other city. Commissar Gadlem thinks this is a case for Breach – the secret police who ruthlessly patrol all cross-border crime. Gadlem thinks someone else should take on the case, and that maybe it's too personal for Borlú… But Borlú insists he can handle it.