The two film "festivals" starting in Boston and Cambridge this week celebrate what are probably the cinema's two most popular genres--the whodunit and the skin flick. In the sixties film festivals tended to showcase a great actor or director, and the nearly constant Bergman and Bogart festivals at places like the Brattle Square are holdovers from those days. Now they tend to focus on particular genres. These festivals aren't Hitchoock festivals or even Radley Metzger festivals; they aim to show the whole range of detective films and erotic films, the good, the bad, and the commercial, the typical and the deviant.

The "whodunit" series at the Orson Welies is probably the most ambitious thing of this kind in be shown in Cambridge in the last few years. It will last two entire months and include 36 feature length films. As with any selection, one can argue with what has been chosen. The management itself, in a small note at the end of its program, regrew that four films--The Maltese Falcon. The Big Sleep, Chinatown and The Conversation--are unavailable. But the group, is so inclusive as it is that the only possible complaint is not what it excludes but what it includes. The recent films--Sleuth, The Long Goodbye , Klute. Harper and The Last of Sheila-- leave you wondering what kind of touch less than-stellar directors like Michael Curtiz had that today's better directors like Robert Altman, don't Sleuth is simply a too-cute stage play turned into a too-cute (and what's worse for a mystery, too easy to figure out) movie: The Last of Sheila and Klute derive their chief interest not so much from their plots as from their settings, an ocean-liner studded with Hollywood stars and the underworld of an urban prostitute. The Long Goodbye tries to take on the whole tradition and do something with the self-consciousness that deliberate manipulation of the audience's expectations allows. The result, inexplicably, was boring.

In a classic Peanuts, Lucy tells Linus about Rosebud as he starts to watch Citizen Kane. Any true whodunit can be ruined that way--it's a foolproof test. The most facile of the genre proceed according to a formula, and if you're good enough--as my mother is with Perry Massons--you can guess the victim, and then, the murderer, nine times out of ten.

But best whodunits, the ones that are more than whodunits, move swiftly along from mystery to discovery, and further on to the last mystery; that even when you add up all the facts you don't know what happened in human terms. At the center of the best mysteries there is always a fake Maltese Falcon, but the true detective gamely takes the next train to Constantinople anyway, to get on the trail of the "real" one.

Coating all this, giving it a hard shiny surface in which we can see ourselves, making the ground under our feet so slippery we can't stop our crazy progression from clue to clue, is an icy cynicism. The detective's ultimate values are never really cynical--if they were he'd be the criminal and not the detective, whether he's Margaret Rutherford playing Agatha Christie's insufferable Miss Marples or Alec Guinness playing Chesterton's quaint Father Brown or Humphrey Bogart as Sam Spade explaining, after Miles Archer's murder, that you have a duty to your partner.

The whodunit doesn't have to--be epistemological, doesn't have to take a position on whether or not we can ever really get to the bottom of what's going on around us. Perhaps the best film being shown at the festival isn't really a whodunit at all. Carol Reed's The Third Man is one of the movies made in the forties that seem to have no flab at all, in which every word, every shot is necessary, not to mention each scene and each character. Holly Martins, writer of westerns and most idealist of the idealist Americans, arrives in Vienna to work for his old friend Harry Lime--who turns out to be dead, run over by his own chauffeur, Calloway, the British military policeman, knows Lime has been mixed up in a vicious black market in penicillin, and Martins undertakes the job of clearing his friend's name and finding his real murderer. Since most mysteries are set in a modern city like Los Angeles or in remote, isolated countryside. Graham Greene's brilliant choice of postwar Vienna is especially impressive--it manages to combine the two; the rootless, warrenlike atmosphere of a metropolls and the underlying terror of long reaches of barren scenery. The Third Man is a film in which the detective story remains entirely secondary. Holly Martins is just as much the naive American in need of protection at the end of the film as at the beginning; as Harry Lime, played by Orson Welles, is lowered into his grave for the second time you feel as if he might at any time reappear, stepping out of the shadows with a silent step more frightening than any sudden gunshot--it's more frightening when the unexpected body you discover is live. The ferris wheel in the Prater will never stop turning--Reed films it from the crazy angles that were Orson Welles' trademark; it is here that he reveals that even the pettiest racketeer has a philosophical motive--and an attractive one at that--for his crimes:

In Italy for thirty years under the Borgias they had warfare, terror, murder, bloodshed--they produced Michelangelo, Leonardo da Vinci and the Renaissance. In Switzerland they had brotherly love, five hundred years of democracy and peace, and what did that produce...? The cuckoo clock. So long, Holly.

Welles becomes a sewer rat; every exit is covered and his seemingly endless ability to scramble in and out of huge blocks of misshapen rubble is exhausted. The underground chase goes on for a little too long--any recent film like Bullitt could give us a more varied and exciting sequence, probably with helicopters and Jensens and a pyrotechnic climax. Major Calloway is a wiser, more upstanding policeman than any (although Ironside may be a close second) but he is part of the irrelevant, whodunit part of the story.

Often the best things about these movies are the people who walk through them, performing little if any plot function. What we finally have to deal with is Anna Schmidt, who comes out of nowhere and walks--with a proper contempt for Martins--further into nowhere, down a wet, leaf-littered road lined by the stumps of trees cut down during the war. Scenes like this are unchanging and final. When Harry Lime suddenly appears on the second story of a bombed-out building, standing in a shroud of a black overcoat, robed and stiff like the ragged statue propped beneath him to the left, he is more than a clue or an answer in a mystery--he is what crawls out from under the rock when the world is destroyed. This is the kind of moment that makes The Third Man worth seeing over and over again.

Whodunits like The Third Man are valuable for what the writer and director manage to accomplish despite their primary goal of creating suspense. The average whodunit (and the Orson Welles festival has its share of those) is almost by definition something you'd never want to see twice--or at least not until you'd forgotten it. But the first time around, anyway, such films may warrant taking out your pipe and putting on you deerstalker cap.