The theme of The Spy Who Came in From the Cold is that espionage is a grubby game that corrupts everyone who plays it, and that our side is just as ruthless, cynical, dishonest and vicious as the other side. Alec Leamas slowly comes to realise all this, and decides he doesn't want to play the game any more.

He also happens to have an English girlfriend who is a Communist. Initially he is scornful of her beliefs, but gradually he comes to see that she believes in something, while he no longer does.