Francois Truffaut's "Confidentially Yours," now at the K-B Janus and Circle MacArthur, is an absent-minded imitation of a late 1940s Hollywood mystery thriller--the genre that French critics dubbed film noir. Truffaut evokes such a pale reflection of the prototypes he's copying that the results might be more accurately described as noir blah.

"Confidentially Yours," derived from a minor American crime thriller, dotes on a resourceful secretary whose flair for amateur sleuthing extricates her boss from a murder rap. Fanny Ardant, the young actress cast as this invaluable Girl Friday, Barbara, is potential dynamite and Truffaut allows her remarkable features--a regal nose set off by a wide brow, wide eyes and wide mouth, always slightly open and capable of projecting expansive, spontaneous smiles--to make an ingratiating spectacle of themselves.

But you can't help feeling that Ardant is being wasted on Truffaut's material, which obliges Barbara to come to the rescue of an employer, a travel agent played by Jean-Louis Trintignant, who seems at once too old, too short and too dense to deserve such a radiant helpmate. The employment and romantic prospects for attractive young Frenchwomen must be severely depressed if a creature like Barbara can content herself with toiling for a boss as lackluster as Trintignant's Julien Vercel while carrying a torch for him, too.

There are discrepancies here that Truffaut doesn't begin to finesse; indeed, the movie concludes with an outrageously self-satisfied in-joke: Barbara gets the boss and appears at the altar prominently pregnant; everyone in the know is supposed to know that the actress really was carrying a child fathered by the director.

It might be easier to share (or excuse) the joke if the movie had emerged as a weightier collaboration, but "Confidentially Yours" is the sort of trifle that tends to give lightweight entertainment a bad name. Nothing connected with the unfolding of the mystery--did drab little M. Vercel kill his unfaithful wife's lover and then add his wife and a third victim, or is he being ingeniously framed?--carries the slightest conviction, and there's not enough variety or misdirection to divert suspicion from the eventual culprit. The mystery is mainly a pretext for Ardant to snoop around in facetious disguises. This cuteness would be even cuter if the director contrived to support it with credible intimations of menace, but he never bothers to rationalize his own plot in either sincere or parodistic terms.

There are two other fetching and amusing young actresses in the cast--Caroline Sihol as Trintignant's slutty mismate and Pascale Pellegrin as a job-seeker who demonstrates surprising speed typing with one finger. Both disappear after brief but delightful and distinctive appearances, suggesting that Truffaut might stand a better chance of renewing himself by finding a suitable showcase for a parade of new faces.

"Confidentially Yours" has been shot in black-and-white to enhance the synthetic pretense of vintage moviemaking, but it's gray-souled, a consequence of linking outmoded plot conventions to a leading man who also seems over the hill. Trintignant seems to be standing in for the director, who also falls well short of demonstrating an adequate appreciation of the formidable young woman who's chosen to link her future with his.