In prison he prepares himself for physical torture, but finds himself in a kind of intellectual duel with his interrogators, his peer Ivanov and the younger Gletkin. Rubashov also talks via coded taps with one of his prison neighbours, a royalist reactionary. And he is haunted by memories of his life, in particular of those he himself has sacrificed on the altar of Party discipline, putting the end above the means.
All of these have a place in the process by which Rubashov, with the more or less subtle encouragement of his interrogators, convinces himself that the dictates of historical logic and Party loyalty demand his own sacrifice. He is led even to public confession to crimes he never committed.
Though the country is never named and its leader is simply "No. 1", the setting of Darkness at Noon is clearly Stalin's show trials of the Old Bolsheviks in the late 1930s. A communist until his recantation in 1938, who had visited the Soviet Union and knew some of its leaders, Koestler was uniquely placed to understand these, with an insider's understanding of both Marxist theory and Stalinist practice. He had also had firsthand experience of imprisonment, by the Fascists in Spain, which is reflected in the convincing details in his depiction of life in prison.
Koestler wanted not only to highlight the brutality of the Stalinist regime but to discredit the ideology which underpinned it. To that end he included some passages of theoretical political speculation, in which Rubashov attempts to use his training in dialectics to make sense of his situation. This is not overdone, however, and remains consistent with the more personal logic of his psychological need for justification.
It is not essential, but the reader will benefit from a basic knowledge of Marxist theory and the history of the Russian Revolution and Stalin's Terror. So I'm not sure I would have appreciated Darkness at Noon properly if I'd read it when it was first recommended to me twenty-five years ago, by one of my high school English teachers.
Darkness at Noon has been widely influential, most obviously on works such as 1984 and A Tomb for Boris Davidovich which also address totalitarianism and the way the Revolution "eats its own". It holds a place as one of the central political novels of the 20th Century.