Venturing into the Russian zone, Greene went up on a Ferris wheel in a bombed-out amusement park. As he looked down at the dots of people far below, he got the idea for the book’s key episode. In the famous Ferris wheel scene, Harry Lime asks an appalled Martins whether he would really care if any of the ‘dots’ on the ground stopped moving. For twenty thousand pounds a dot, Lime believes that anyone would be happy to wipe out one dot after another. In this chilling scene, Lime is a twentieth-century equivalent of the devil’s ambassador, who takes Martins to a high place to survey the world, alternately threatening and tempting him. ‘Nobody thinks in terms of human beings,’ Lime says in justification of his penicillin racket. ‘Governments don’t, so why should we?’ These cynical lines are spoken in the shadow of the industrialised killings of Treblinka and the blinding flash of Nagasaki, which had brought unprecedented destruction. Whatever his detractors might say, Greene was a moralist troubled by human turpitude and evil in our time. In an article for the Catholic journal Tablet in 1951 he wrote: ‘Today the human body is regarded as expendable material, something to be eliminated wholesale by the atom bomb, a kind of anonymous carrion.’

The book differs in many significant ways from the film. Carol Reed was courageous enough to give his movie a determinedly unhappy ending, which even the pessimistic Greene was reluctant to endorse. Reed stuck to his conviction that Lime’s girlfriend, the political refugee Anna Schmidt, would not have any cause to love the man who had just shot him. So the film ends with Anna walking imperiously past Martins as he waits for her in the cemetery after Lime’s burial, not even pausing to acknowledge him. It is one of the great moments in screen history. Other changes were made with a view to box-office profit. In the book Anna is a physically unremarkable woman. (‘It wasn’t a beautiful face,’ Greene writes of her. ‘It was a face to live with.’) But in the film Anna is played by the mournfully beautiful Alida Valli, whose stylish reserve belies a history, one suspects, of Central European terror and Stalinist persecution. The film makes much of Anna’s fatal love for Harry Lime and the irresistible attraction she holds for the lovelorn Martins. When Martins moves his hand to her lovely face in the film, Anna turns away: she has no need of this ‘romantic fool’ who brings her flowers at midnight and drinks too much.