Graham Greene belonged to the first generation of British writers who grew up with the movies, and his work, like that of his contemporaries, Evelyn Waugh, Henry Green and Christopher Isherwood, was deeply influenced by the new medium. What Greene learned from cinema was how to hold his readers in the coils of a suspenseful plot while exploring unsettling moral and metaphysical themes, and how to evoke character and milieu with the verbal equivalent of cinematic close-ups and pans. His literary imagination, however, did not easily translate in the opposite direction. Nearly every novel he wrote was eventually made into a film, but very few have been good films, and only one is a classic: The Third Man (1949), directed by Carol Reed, which Greene scripted himself.

It was the success of The Third Man that prompted Greene to propose soon afterwards another film that would have been very similar in style and substance. No Man's Land is the "treatment" Greene wrote for that film, which was never made. Like The Third Man, it is a mystery thriller set in occupied Europe a few years after the end of the second world war and early in the cold war. Like The Third Man it involves the quest of one male character for another to whom he is emotionally attached, in circumstances of extreme jeopardy, with the additional spice of female love interest. In both stories the personal and the poli-tical are entwined thematically as well as in the narrative. What is distinctive about No Man's Land is the religious and specifically Catholic strand in the story.

A film treatment is usually a detailed summary of what the proposed film would actually show. Greene's treatments are formally indistinguishable from his short stories, and include many touches that would be impossible to replicate in the film medium. For example: "... before Brown had reached the door Starhov was already asleep, like the dead, like the effigy on his own tomb, Brown thought, except that there would be no effigy on the tomb of one who had committed the crime of trust".

Conceivably, a carefully composed shot of the sleeping Starhov could evoke the effigy simile, but there is no way that Brown's paradoxical and very Greeneian thought could be conveyed visually. Greene made explicit in his treatments things that would become part of the subtext of the projected films, or disappear altogether.

"Trust" is a keyword - the keyword - of No Man's Land. The cold war is the political expression of profound mutual mistrust between the Soviet bloc and the western powers, and has given rise to an elaborate system of espionage, the world of the double agent and the double-cross, which fascinated Greene from adolescence onwards. Stahov's "crime" in trusting Brown's word is political, just as Brown's duty to betray him is political. The political is opposed to the moral. "If we loved God do you think all this would exist?" Clara asks Brown, meaning by "all this" the violence and treachery and futile conspiracies of the cold war.

The relationship between Brown and Clara, the inconstant German Catholic with whom he falls in love, sometimes seems sketched rather than fully realised, but it might have been more convincing when acted out on screen. Also it is relevant that religious belief and sexual promiscuity were improbably combined in Greene's real-life mistress at this time, Catherine Walston, with whom he fell in love at their first meeting. James Sexton, who has edited No Man's Land for publication, is absolutely right to see it as driven by the novelist's jealous and obsessive passion for Catherine, who was later to serve as the model for another, more complex and believable heroine, Sarah of The End of the Affair. The hero's name, Brown, seems to be a teasing reference to the author's.

If the scenes between Brown and Clara sometimes come dangerously close to self-parody, The Stranger's Hand actually began as a conscious exercise in self-parody; but in many ways it is a more controlled piece of writing and it is a pity it is unfinished. The first two paragraphs were submitted by Greene under the name of N Wilkinson for a New Statesman competition in May 1949 to write the best imitation or parody of the opening lines of a novel by any writer called Green or Greene. His entry was awarded the second prize. His friend, the Italian film director, Mario Soldati, persuaded him to continue the story in earnest as a film treatment, but Greene apparently lost interest, and another hand eventually brought it to a conclusion so that the movie could be made. But the sizeable fragment of the original treatment is first class.

Greene, who wrote memorably about his own alienated childhood, was always skilful at creating unhappy, sensitive children, and the plight of the eight-year-old Roger Court, posted like a parcel to a strange foreign city, Venice, to meet a long-absent father who fails to turn up, is vividly and movingly portrayed. The rituals with which he seeks to distract himself, like the improvised game of table-cricket, and the moments when his courage and self-control suddenly give way to helpless tears, are beautifully judged. Here, as in his story "The Basement Room", which he adapted very successfully for the cinema as The Fallen Idol (1948), Greene perhaps took a leaf out of Henry James's book, specifically from What Maisie Knew, in rendering a naive point of view in a well-formed and eloquent style, so that we apprehend both the child's innocence and the seriousness of what is at stake in the adult world.

There are several similarities between this tale and No Man's Land. Again the background is cold-war espionage and intrigue; again confrontation across the frontiers of disputed territory serves as a metaphor for moral and emotional disconnection: "He turned on the bedside light to read his letter ... 'Dear Roger' (the phrase had the same distance as 'my father': they seemed to be signalling to each other tentatively over the No-Man's Land of two years, a waste filled with the wreckage of other lives than their own)."

Just as Greene's jealously possessive feelings about Catherine Walston provided much of the emotional fuel for No Man's Land, so (one might speculate) did his guilt about deserting his own children, when he separated from his wife Vivien, find expression in the compassionate portrait of Roger, and in Major Court's stoical sense of having failed as both parent and policeman. The way Roger uses the juvenile adventure stories he reads as a way of interpreting his own experiences parallels the intertextual allusions to Turgenev in No Man's Land. Both treatments exhibit Greene's incomparable ability to evoke the sense of place,.

These two texts were written between The Heart of the Matter and The End of the Affair, when Greene was at the height of his powers as a novelist. They are minor works, but Greene is one of the select group of modern writers of whom it can be said that almost nothing they wrote is without interest or evidence of a unique literary gift.

· Adapted from the foreword to No Man's Land published by Hesperus Press this month. To order a copy for £7.99 with free UK p&p call Guardian book service on 0870 836 0875. David Lodge's latest novel is Author, Author published by Penguin, price £7.99.