The book too place during the 1930's. A good review for it is HERE
If twentieth century literature is to be remembered for anything in particular, it is likely that novels like Darkness at Noon will be considered typical of one of its major distinctive themes. Like other great works of this century, it deals with the relationship of the state and the individual. There is perhaps an inherent similarity between these tales of dehumanisation and despair; and few people would want to be stranded on a desert island with such depressing fare as provided by Kafka, Orwell, Solzhenitsyn, and Koestler.
The parallels between Darkness at Noon and 1984 are particularly strong, both dealing (at least in large measure) with the interrogation, torture and breakdown of one man by another. Koestler - who wrote earlier, just before the Second World War - even goes so far as to remark that the one thing the totalitarian state does not do to maintain the illusion of its own righteousness is to doctor the back issues of its newspapers, which is of course Winston's occupation in 1984.
While 1984 is set in a fictional dictatorship, Darkness at Noon is set in the real world contemporary with its date of writing. The main character, Rubashov, is fictional, but he is a companion of Lenin from the days of the Russian Revolution who has becume disillusioned through the rule of Stalin. (Neither Lenin nor Stalin is given his real name, but it is clear who is represented by both "the Founder" and "No. 1".) Rubashov's main problem is with the cynical traty between Stalin and Hitler, and the way that this seems a betrayal of the Communist cause, not just to him but to members of the party cells that he works with outside Russia. His time in prison and the torture he undergoes - sleep deprivation, interrogations at unexpected times, refusal of medical treatment, bright lights and so on - makes him think about his life, his dedication to Communism and where it all went wrong.
In short, Darkness at Noon is a perceptive novel, though a depressing one. Like all this style of work, it is about human nature under great stress, and uses this to show us things about ourselves.
Having written the main part of this review before discovering the existence of the allegations about Koestler's life (basically, that he was a serial rapist), I was in two minds about whether I should put it up. In the end, I felt that this novel was a major piece of work, whatever the personal life of the author. I do not condone his actions, which hardly seem to have been those of a normal member of society. I end up dithering between the point of view that his private life and his art are separate things, so that one can be ignored while enjoying the other; and the view that there is a close connection between them, and that enjoying one is condoning the other.