The book begins with a crime being investigated in two parallel universe cities. The two cities are divided and segregated, and no crossing over is permitted, and this is controlled by government officers known as The Breach.
A murder of a young woman is discovered in one of the cities, Beszel; the women was a member of the parallel city, Ul Qoma. The woman, Mahalia Geary, had made enemies due to her insistence on the discovery of a third, lost city, known as Orciny, which lies between the borders of the other two cities.
Inspector Tyador Borlu leads the case and finds himself in the middle of a political conspiracy between the two cities. He realizes that her murder was instigated because she had pondered the thought of a hidden city that both governments are tying to cover up. The book ends with Borlu wondering whether more cities like this exist.
