If Luis Bunuel had not directed “Belle de Jour,” it probably would have turned out to be nothing more than a case history from the pages of Krafft-Ebbing. On the very surface it merely tells the story of how a wealthy married woman sets out to solve her “abnormal” sexual hang-ups.

Since she is a masochist, who secretly yearns to be dominated, debased and sexually abused, her life of complete comfort leaves her cold. For relief, she frequently resorts to erotic daydreams, but finally the moment comes when she is compelled to act out her sexual fantasies. After a couple of false starts, she becomes a Belle de Jour, an afternoon prostitute, in a small middle-class brothel.

There, as might be expected, her masochistic desires are more than satisfied. She even takes on a Japanese businessman who comes in carrying a strange black box that terrifies the regular prostitutes. But regardless of how she suffers (or enjoys herself), she returns home each evening in time to greet her gentle, unsuspecting husband.

However, “Belle de Jour” is a work of art, not a clinical record. And as a work of art it is as much concerned with fantasy as it is with reality. The basic plot (as bizarre as it may be) merely acts as an armature for the film proper. What goes on around the plot is infinitely more important than the plot itself.

Bunuel’s relaxed, sure-handed directorial style is mainly responsible for the film’s success. A picture such as “Belle de Jour” must be put together like a house of cards—one slip and it would have been a disaster. The director’s first task was to make the film believable. This was accomplished principally through flawless casting and meticulous characterization.

By the time that Severine (played brilliantly by Catherine Deneuve) becomes a prostitute, she is no longer an actress; step by step, and detail by detail, the audience is convinced that what is happening on the screen is an actual event. In fact, the level of reality is so high that one is unprepared for the gradual reappearance of fantasy.

For example, when Severine takes up with a comic book-like gangster near the end of the picture, one is tempted to accept the episode at face value. But there is a better than even chance that part, if not all, of the episode is merely a figment of her erotic imagination. The gangster, complete with scars, gold teeth, purple socks and patent leather boots, is more a walking caricature than a human being.

But if the stereotype thug is real (and he might be) it only serves to prove that we have no reliable way to measure the difference between fantasy and reality.

The picture’s rather puzzling conclusion suggests that either Severine has found eternal guilt more satisfying than physical pain, or that she has finally discovered the ultimate masochistic fantasy.

However, the main difficulty in understanding or accepting the ending is that it presents a total inversion of traditional values—evil is rewarded, and virtue is punished. (Maybe.) But regardless of how ambiguous the theme may be, “Belle de Jour” is without a doubt one of the best foreign films I have ever seen.

Its seamless, overall quality makes it quite clear why it was named Best Picture at the Venice Film Festival. It is one of the few award-winning motion pictures in recent times to live up to its reputation.