"The Third Man" (1949) is so elegant, tiny and perfect that it feels more like a watch than a movie: It should have been directed by Patek Phillipe.

But it was directed by the Britisher Carol Reed, who got himself into the masterpiece category this one time out in a career that was otherwise respectable, if never quite brilliant (his second-best film was probably "Odd Man Out" in 1947; his worst, "Oliver!" in 1968). But he had great collaborators: a script by Graham Greene, and a cast that included Orson Welles at his most Wellesian, Joseph Cotten at his most Cotteny, Trevor Howard at his most Howardian (you could land a plane on that stiff upper lip), Alida Valli at her most coldly beautiful (forward the slight brigade, into the Valli of indifference!) and, best of all, Vienna at its most Viennese.

Vienna, Vienna, that strudelin' town! What a haunted burg it is in the aftermath of the last war and on the cusp of a cold one: dank, shadowy, medieval, shot through with ruins, patrolled by MPs in jeeps, full of shadows, alleys, tunnels and sewers, the film noir city viewed through the fractured prism of European existentialism and Graham Greene's weary, bitter aversion to the possibility of love.

But despite its justified fame and worldwide success, "The Third Man" has not been seen by most Americans in its true form. That's because co-producer David O. Selznick decided it was a tad slow for audiences this side of the drink, so he commissioned a Yank version.

The result was hardly butchery, more like a deft tummy tuck, and it might have even improved the film by speeding it up just enough for us children of the Republic of Attention Deficit Disorder who even then were chronically unadaptable to the more languid pace of the European product. On the other hand, here's a chance to judge for yourself, as a newly restored version of the European version--it's 11 minutes longer--opens today at the American Film Institute for a week.

Naive American cowboy writer Holly Martins (Cotten) arrives in Vienna to find out that his best pal, Harry Lime (Orson Welles), has just died in a mysterious accident. Discovering anomalies in the various accounts of Harry's death, and running into barely concealed boredom from British military cop Calloway (Howard), Holly vows loudly to investigate.

But his naive hubris merely stirs the maggots that festered in Harry's wake. Hoping to vindicate Harry by finding his murderer, he finds instead that Harry is a murderer. At the same time, he falls in love with Harry's girlfriend, Anna (Valli), and even as he discovers there's something about Harry (namely, that he isn't dead), he learns there's something about love: namely that the right woman sometimes falls in it with the wrong man.

What connoisseurs will notice right away in this restored version is the scene-setting prologue being read, not as it was in the American version by Cotten in his tweedy, dithering prep-school voice but in the jaded, sophisticated voice of an Englishman, rather amused at the tale he's about to tell. As well he should be, since the voice is Reed's own. Very interesting. Still, it's better with Cotten, because he is, after all, the point-of-view character, the first-person "I" of the movie. If Reed reads it, who's talking? It lacks an organic connection to the story.

Other changes are small. Scenes are held slightly longer; tidbits of dialogue are added; Holly's tragic wait for Anna under the weeping elms in the cemetery is longer, so that it's not a dirge but an ordeal. And the famous chase through the sewers at the end is slightly lengthened. This last touch is the one heartily approved of here: This whirligig of dazzle, comparable to and even the equal of Welles's famous funhouse gunfight at the end of "The Lady From Shanghai," is possibly the best chase ever put on film, as the forces of law and order hunt the heretofore imperturbable Harry Lime like a rat through the sewers.

And Welles! Has an actor ever made himself more vivid in less time? The first 10 reels are pure setup, so he'd better deliver, and he does. He is on screen for probably less than five minutes, yet he dominates the action totally. His face somehow both taut and plump at once, his eyes merry with ironic detachment and amusement at the mess into which the world has plunged itself. He's quite a piece of work, gentleman scoundrel as mass murderer of children. He makes sociopathy seem like a witty minor vice, akin to putting two olives in a martini instead of one. And he seems to relish his chosen profession even if it causes him a little heartburn. "Please remember to bring the tablets, old man" he tells Holly, as if the world's problems could be relieved by a R-O-L-A-I-D-S.

The famous scene on the Ferris wheel, as Harry points out to Holly all the little people moving around far below and then postulates the calculus of relativism: "If I said to you, you can have 20,000 pounds for every dot that stops, would you really, old man, tell me to keep my money, without hesitation, or would you calculate how many dots you could afford to spare?" It remains one of the most coldly chilling, yet charismatic, few seconds in movie history.

It's a movie of great scenes--the dying wiggle of Harry's fingers up through the sealed grating as the cops close in, the fat little boy with the ball who denounces Holly as a killer, Anna's utter indifference to Holly's twisted declaration of love, Calloway's bitter contempt for an amateur. But it's not just scenes. It has a unity of vision and a coherence of theme and a perfection of tone that are rarely achieved in movies or any stories, for that matter. To see it is to learn anew just how great a movie can be and what bliss 90 minutes alone in the dark can provide.

The Third Man (104 minutes, at the American Film Institute's National Film Theater at the Kennedy Center through July 11) is unrated; it has implied violence, a mild gunfight and a great deal of tension.

CAPTION: Orson Welles is onscreen less than five minutes, yet he dominates the action.

CAPTION: Joseph Cotten and Orson Welles in the restored version of "The Third Man" at the American Film Institute.