"On Golden Pond," a movie that encourages us to derive sentimental gratification from counterfeit assurances, opens with a lofty photogenic downbeat -- an aerial view of a placid body of water reflecting a luscious golden tint. This warmly inviting image (of Big Squam Lake in New Hampshire) may seem especially gratifying to snow-weary audiences. But unfortunately, the vicarious appeal of the balmy setting is promptly tarnished by the arrival of human nuisances, an elderly married couple named Norman and Ethel Thayer, whom their creator, playwright Ernest Thompson, is determined to pass off as exemplary Old Dears. The evidence supplied by Thompson, also responsible for the screen adaptation of his play, is consistently disillusioning.

The doting phoniness of the text has probably been aggravated rather than improved by a formidable casting coup -- uniting Henry Fonda and Katharine Hepburn for the first time in their illustrious careers and creating the shallowest heartwarmer in recent memory. While it's painful watching a frail Fonda shuffle through a role as dubious as Norman, an old crank of an academician whose sarcasm is first presented as evidence of a cold heart and then as an endearing quirk, he doesn't disgrace himself. At least there's some restraint in Fonda's approach to Norman's gratuitous set of wisecracks and wobbles.

Hepburn, on the other hand, enters quavering and gushing. "The loo-oons, the loo-oons! Oh, Norman, they're welcoming us back!" These loons, a devoted couple, according to Ethel, function with rather ludicrous explicitness as symbols for the Thayers. In a suspense episode added for the movie a dead loon clearly foreshadows peril for Norman. Also, it's meant to be a measure of Norman's mellowing that he begins this summer vacation grumbling "I don't hear anything" when Ethel calls attention to her beloved birds and exits exclaiming, "Ethel, the loons!"

I don't mind a comforting sentimental plunge every so often, but "On Golden Pond" presumes far too smugly and ineptly on moviegoing good will and nostalgia. Thompson appears to have a flair for the facetious that might serve him better in the knockabout farces of the theater. As a tenderhearted humorist, he seems a transparent fraud. There's not a single character trait or dramatic situation of alleged consequence in "On Golden Pond" which is not exposed sooner or later as an arbitrary tease, destined to be forgotten.

For example, the ostensible crisis is the arrival of the Thayers' grown daughter, Chelsea (Jane Fonda in a dreary minor role), who supposedly has reason to feel that her father has never loved her, on the occasion of his 80th birthday party. Although the alienation theme is set up, it's deflected by a plot detour that suggests Chelsea has a lot of gall rather than a sympathetic case. She arrives at the cabin on Golden Pond with a new fiance, Dabney Coleman as a far-fetched, humorless L.A. dentist named Bill, who seems to have gotten his brains scrambled at an est seminar, and a sassy teen-ager, Doug McKeon as Bill's son Billy. Although it's been eight years since Chelsea has paid the folks a call, she asks if they'll look after Billy for a month while she and her boyfriend go to Europe.

Billy looms as a potential problem for about 30 seconds. His father deplores what's become of him under the supervision of his unfit mother and solemnly vows to "eradicate some of the dishevelment," right before leaving the kid with elderly strangers. Whatever delinquent behavior Billy practiced back in L.A. is no match for the wholesome influences of Golden Pond. At Norman's insistence he begins reading good books and with Norman's advice he becomes a crackerjack fisherman.

With this noncrisis resolved, Chelsea eventually returns, supposedly a newlywed but without the groom. "He went back to the Coast; he had a mouth that needed looking into," she confides with a mysterious straight face. Seeing the camaraderie between her old dad and brand-new foster son, Chelsea throws a brief temper tantrum, provoking Hepburn's most excruciating flutters: "You are a big girl now! Aren't you tired of it all? You have this unpleasant chip on your shoulder which is very unattractive. You only come home when I beg you to, and when you get here all you can do is be disagreeable about the past. Life marches by, Chelsea! I suggest you get on with it! You're such a nice person. Can't you think of something nice to say?"

Needless to say, Chelsea feels thoroughly ashamed of herself, and a lifetime of misunderstanding seems to melt away when she approaches Norman and shows him how she has finally mastered the back flip, a dive that always frustrated her in girlhood. The source of Thompson's shallowness may be revealed in Ethel's misconception of Chelsea as a nice person whose resentments can be healed if only she'll start saying nice things to everybody. The playwright shares a similar misconception of the innate virtuousness of his characters, especially those would-be ingratiating old crocks Norman and Ethel.

Thompson is also a bad joker, and he bottoms out in a sequence where Norman's apparent coronary is revealed to be one more fake-out. Ethel's pet term of endearment for Norman is "you old poop!" and this nifty is even reiterated when she fears that her husband may be at death's door: "Dear God, you don't want him now! He's just an old poop!" Until something even more mirthful crosses your path, "On Golden Pond" may be considered the latest refinement in poopy domestic comedy.