Extravagantly overpraised as a novel, Jerzy Kosinski's "Being There" continues to enjoy a charmed life as a movie.

The fragile satric fable seemed to defy adaptation. But despite its shortcomings, director Hal Ashby managed to transplant the undernourished narrative with remarkable success.

Chance, played by Peter Sellers, is a retarded founding who has grown to middle age tending the garden of a wealthy benefactor who resides in Washington. The benefactor dies, and Chance is forced to fend for himself in the outside world, which he knows only through the images he has absorbed on television. He blunders into the protective custody of a prominent couple, Melvyn Douglas as a dying finacier and industrial named Rand and Shirley MacLaine as his affectionate spouse.

Eager to be agreeable, Chance guilelessly ingratiates himself by smiling a lot, reiterating other people's remarks in a blandly flattering tone of voice and recalling commonplaces about gardening. These are ministerpreted as sagacious allusions to the State of the Union by his kindly, influential acquaintances, who include Jack Warden as Rand's perplexed, impotent best Friend, the president of the United States.

For example, when Chance remarks, "There is much to be done in the winter -- the seeds must be prepared for the spring" or "As long as the roots are not severed, all is well and all will be well in the garden," it is presumed that he is speaking figuratively about great national problems that preoccupy men like Rand and the president. In fact, Chance is merely making polite conversation to the best of his feeble-minded ability.

In a similar respect, Chance earns the reputation of quite a wit within the Rand household. After riding in an elevator for the first time in his sheltered life, Chance can't help observing, "That's a very small room." The butler accompanying him laughs and agrees, "Yes, the smallest in the house." The butler spreads the word that the master's new friend has a disarming sense of humor and thereafter anticipates his accidental nifties. Even the name that Chance asumes with the Rands -- Chauncey Gardiner -- is the result of a misunderstanding, originating in a sudden fit of coughing when he first introduces himself to Mrs. Rand.

Chance survives on blind instinct and dump luck, securing a position of trust and affection with rich, powerful patrons. Ultimately, it appears that the fate of the nation may be entrusted to Chance, the pun obviously intended. "Being There" closes with a sublimely witty image of this fortunate dunce literally walking along the brink.

The suspension of disbelief necessary for Chance's absurd progress -- and the filmmakers' comic tableaux and incidental joking while he sleepwalks to the top -- is maintained as long as be functions within hermetic settings: his original home, Mrs. Rand's limo and the Rands' palatial residence. The illusion begins to decompose when we're asked to believe that Chance could appeal not only to the susceptible Rands, but also to a widening circle of their associates and finally to a vast television public.

Sellers' performance, a tightrope act of enigmatic stupidity that seems to combine the persona of Stan Laurel with the lobotomized character played by Robert Shaw in the film version of "The Caretaker," looks dreadfully exposed when Chance goes on a TV talk show.

Sellers tries to sustain a single-key performance, but he seems in and out of the appropriate montone. Sometimes he catches the ideal ambiguous reading, making it impossible to tell from the outside whether Chance is sage or idiot. Just as often the idiot clearly gives himself away.

MacLaine and Douglas are delightful as the Rands, who are fools of a kind. Their performances seem both witty and unerring, although I could have done without the lugubrious facetiousness of the big comic set piece in which MacLaine supposedly ends up gratifying herself after trying to seduce Sellers away from an installment of "Mister Rogers." This episode might be usefully contrasted with the sequence in "Joe Tynan" where Rip Torn is discovered in mid-dalliance as textbook examples of, respectively, lewd comedy that goes astray and lewd comedy that works like a charm.

Richard Dysart gives the film's unsung astute performance ad Rand's doctor, who suspects the awful truth about Chance but declines to shatter his dying patient's illusions. This character gives the material a crucial realistic underpinning: despite the general fantasy that surrounds Chance in the Rand household, an accurate assement of his character is not impossible.

A fastidious absurdist farce about the career advancement of an amiable cipher may have a special snob appeal in Washington, where Chance can be conveniently identified with hosts of servile attendants-to-power. It's also possible that the material will strike a nerve by expressing a widespread belief that the country has become leaderless, with no apparent relief in sight.

There's something in this apprehension, but the comic potential is severely limited to the dry, narrow ironic vein that Kosinski and Ashby feel comfortable exploiting. Less rarefied and more convival spirits might take the same theme for a merrier ride. Ashby's reputation as a notoriously earnest and laid-back director isn't undeserved. Everything that works in "Being There" seems to work as the result of a great deal of evident contemplative deliberation.

Still, Ashby and several deft collaborators, notably the superb young cinematographer Caleb Deschanel, who has already distinguished himself in the past half-year with "More American Graffiti" and "The Black Stallion," sustain a droll, impeccably deadpan depiction than can be appreciated as a triumph of pictorial style and tonal consistency even if one finds the satiric substance trifling or unconvincing.

Under the circumstances "Being There remains a rather overspecialized, thin blooded tour de force, uniquely cerebral for an American comedy aimed at a popular market. The problem with an achievement of this sort is that it's easier to admire "Being There" than to enjoy it.