Inspector Tyador Borlú, of the Extreme Crime Squad in the European city-state of Besźel, investigates the murder of Mahalia Geary, a foreign student found dead with her face disfigured, in a Besźel street. He soon learns that Geary had been involved in the political and cultural turmoil involving Besźel and its "twin city" of Ul Qoma. His investigations start in his home city of Besźel, lead him to Ul Qoma to assist the Ul Qoman police in their work, and eventually result in an examination of the legend of Orciny, a rumoured third city existing in the spaces between Besźel and Ul Qoma.
When the body of a woman is found in Bulkya Docks, an area that sits on the border between Beszel and Ul Qoma, Inspector Tyador Borlú notices similarities with an old case that has always haunted him.
After attempting to illegally crossover into Ul Qoma, Mr Geary is taken by Breach and swiftly deported from Beszel with his wife. Borlu is desperate to know who Mr Geary was planning to meet, but it is out of his hands.
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 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.