Postcard from Turin: The stranger's hand: Ian Thomson met Mario Soldati to learn more about Primo Levi. Instead, the conversation was entirely dominated by a third man

TALKING to Mario Soldati is a struggle these days. Almost 90, the veteran Italian novelist and film director suffers from a language disorder called nominal aphasia. Periods of lucidity are sabotaged by difficulties in finding the right word.

But I was interested in meeting Soldati, and finding out if he could help with my biography of Primo Levi. Like Levi, he was born in Turin. And while rummaging in the archives of the city newspaper, La Stampa, I discovered a curious letter from Levi, dated March 1978. It begins with a formal 'Dear Soldati', and then goes on to reminisce about a drinks party in Venice and the unspoken pride the two took in addressing one another in the formal style (while everyone else insisted on using the familiar tu).

Educated in Turin by Jesuits, Soldati comes from partisan Piedmontese stock and is rarely seen without a dog-tooth tweed jacket and Toscano cigar. He appeared faintly dejected when I met him at his home near Turin. His memories of Primo Levi came in snatches and he kept confusing the author of If This is a Man with Graham Greene. 'Did I tell you how my friend Graham took me to an opium den?' Soldati motioned his smouldering cigar to a photograph of Greene taken in Capri. 'Sierra Leone that was, Christmas 1967 . . .' I could discern little logic in what Soldati was saying until he suddenly announced: 'I once made a film with Graham. In Venice] What was it called? Help me, Gina]'

Soldati's secretary replied with crisp efficiency: 'The film was called La Mano dello Straniero. Mr Greene wrote the treatment and agreed to act as your associate producer.' She added: 'It came out in 1954.'

'Gina, you are a genius.' Soldati waved his walking stick in the air. The Stranger's Hand is the legendary, long-lost suspense thriller that starred both Trevor Howard and Alida Valli from The Third Man. Directed by Soldati himself, this Venetian film was Greene's first appearance on screen. Or rather, his hand's: it appears in close-up undoing the knot of a fireboat on the Grand Canal.

'My movie is completely forgotten now.' Soldati tapped the ash-tip into the palm of his hand. 'And in a way I prefer it like that - as something secret between me and Graham.' He gave me an impish smile: 'You know the film began as a practical joke?'

In May 1949 the New Statesman had run a competition which offered a prize of one guinea for the best parody of a novel by any writer named Green or Greene. A certain N Wilkinson (none other than Graham Greene) came second with an entry entitled The Stranger's Hand: An Entertainment. There were only two paragraphs, but Soldati was convinced they had the makings of a film about Yugoslav agents up to no good in post-war Venice.

The Stranger's Hand was favourably reviewed by Dilys Powell (she said it was better than all the other films then showing). By that time, though, Marshal Tito of Yugoslavia had split with the Soviet Union and become, in Graham Greene's words, a 'white-headed boy of the West'. Unfortunately the film was about a Yugoslavian kidnapping of a British officer. 'This didn't do us any good at all.' Soldati lit another cigar. 'In a way it looked as though we had made Tito the villain of the piece.' And The Stranger's Hand sank without trace.

'Tomorrow night I shall give you a private viewing,' said Soldati as he lifted himself from his chair with the walking stick. 'I think it is my best movie.' Certainly The Stranger's Hand did not lack for distinguished help. Music was composed by the great Nina Rota (who had provided many scores for Fellini); and the novelist Giorgio Bassani (author of The Garden of the Finzi-Continis) was hired for advice on authentic Venetian dialect.

Gina set up the projection screen the following night. The moment the film began to roll in flickering black and white, Soldati made a megaphone of his hands and yelled: 'Silenzio] Silenzio]' 'Calm down, Mario,' Gina said. 'You're not a film director any more.'

Soldati was now only the glowing tip of a cigar in the dark. 'Venice has never looked so . . . real,' he sighed. The camera panned across peeling palaces and rotted gondola posts, then cut to chocolate ice cream at Florian's. The treatment itself is authentic Graham Greene: 'Major Court stood at the locked window and watched far below a gondola pass by, laden with ash cans. It was early morning and the hour for sewage.' The very British intelligence officer is played, naturally, by Trevor Howard, Greene's favourite leading man. He has just been bundled aboard a Yugoslav ship which is about to ferry him behind the Iron Curtain for interrogation. 'Would you mind rolling up your sleeve, Major Court?' asks his kidnapper, syringe in hand.

The suspense is terrific, very Eric Ambler. ('Don't worry. Nobody can disappear in Venice,' says an Italian police commissioner. 'If he is in Venice.') The Stranger's Hand gallops along to a rousing finale when the enemy ship is pursued by one of the gleaming red speed boats of the Venice Fire Brigade. 'Stop the film, Gina,' ordered Soldati. And there it is, Greene's hand untying one of the brigade gondolas.

A gun battle ensues, after which Trevor Howard is brought ashore and reunited with his son. 'Complimenti, Mario,' said Gina, 'you have made a marvellous film.'

I left Mario Soldati laden with his novels, all of them inscribed by the author. One of them - The Capri Letters - carried a message: 'Dear Ian: Sorry I could not be of more help with Primo Levi. I did meet him in Venice, but why or when I can't remember. Forgive me. Venice means only one thing to me now: my dear friend Graham, who died too young.'

The treatment of 'The Stranger's Hand' is published in 'Mornings in the Dark: The Graham Greene Film Reader' (Carcanet pounds 29.95) edited by David Parkinson.