When it was first released, back in 1967, it was described as pornography. And no wonder. In it, Severine (Catherine Deneuve), wife of good doctor Pierre, daydreams that her husband punishes her for frigidity by having her tied, gagged, flogged and raped by his lascivious coachmen. She then goes each afternoon to a clandestine brothel, under the name of Belle de Jour, where she is hired by an elderly aristo for some arcane perversions.

It is the work of Luis Buñuel, the great Spanish director, whose first film, Un Chien Andalou, made with Salvador Dali in 1928, delighted the Surrealists and was a succës de scandale. Right up to the end of his life, in 1983, the man never ceased to shock, surprise and produce riddles full of moral ambiguity. Many film-makers regard him as the greatest of the great, the gentleman-devil of the cinema.

Belle de Jour is the highlight of the first part of the National Film Theatre's Buñuel retrospective. The film turns France's finest female star into a masochistically erotic figure. Severine is a part only a very brave actress would contemplate - yet Buñuel steers the film away from pornography. It is provocative in an entirely unsalacious way.

It is also every bit as surrealist a tract as Un Chien Andalou, and it whacks the bourgeoisie every bit as hard as The Discreet Charm of the Bourgeoisie did many years later. It doesn't distinguish between fantasy and reality: it says they are often the same thing. That's why Guillermo Del Toro, whose Pan's Labyrinth is still running in London, has been dubbed a latterday Buñuel.

But Buñuel was the master, capable of making great films with no money in Mexico, with rather more dosh in Spain and with most of what he wanted in France. He just kept renewing himself, and the result was often a film that would make the jaw drop.

Yet if you look for spectacular filmmaking in Buñuel, you'll almost certainly be disappointed. Sometimes they seem as plain as a pikestaff. This is why some young would-be film-makers can't see why we make such a fuss of the man. The trick is to look beneath that self-effacing style.

Belle de Jour is one of the most extraordinary examples of that style, its incendiary story dressed in the simplest of technical clothes. You could call this artful simplicity. But it sure carries more weight than most of the million-dollar entertainments of today.

The NFT season, which starts tomorrow and concentrates on his early work, is augmented by the release of a new eight-film box-set of his work from Optimum (£44.99) which includes Belle de Jour, That Oscure Object of Desire, The Discreet Charm of the Bourgeoisie and Tristana. About time too, since these are classics that will never be bettered.