Very original story, "The Rules of Marriage." Michael Hagen is a successful writer approaching middle age with a sprawling, luxurious home in Connecticut and a beach house on Fire Island; a lovely wife, Joan, with whom he goes to the city to meet sophisticated friends; and two smart, sensitive kids, Margo and Charles. But something's terribly wrong. Michael and Joan have fallen out of love. They're indifferent. They're unfaithful. They're hurt and confused. The kids are hurt and confused and . . .

Sound familiar? You got it. It's CBS' made-for-TV version of "Shoot the Moon," only Elliott Gould replaces Albert Finney and Elizabeth Montgomery (without her twitching nose) stands in for Diane Keaton. The most significant difference between them is that it takes "The Rules of Marriage," at 9 tonight and tomorrow on Channel 9, four hours to tell the painful, soggy tale of contemporary, upper-middle-class America.

You can't blame the network for imitating a well-received movie; television isn't exactly brimming with good ideas these days. Besides, it's ratings sweeps month. But apparently there's no shame in pilfering just about everything--from the uncomfortable celebration dinner that opens the plot to the husband's occupation and his seashore fling with a hip young thing who looks as if she just stepped out of a Ralph Lauren ad. It even has a touch of Woody Allen: Throughout the story, family members suddenly face the camera and speak to the audience, to say what they're really thinking. For example, Joan confesses: "We exchanged a juicy kiss this morning after fervent toothbrushing . . . I wanted more giggle . But I always do. But I never say it. Except to strangers. You."

And four hours? Early on, "Rules" has its moments. We find out, at least, why the marriage has gone sour--something we're never sure of in "Shoot the Moon." Joan disapproves of Michael's hack writing and wants him to create "a beautiful novel," and Michael resents her lack of interest in his work. The best (and most original) scene is when Michael discovers Joan is having an affair with his divorced best friend, the family dentist, Alan Murray (Michael Murphy). He learns it from his 13-year-old daughter, Margo, sensitively played by Susan Blackstone, who protectively won't tell him why she has a pair of her mother's earrings. When he presses her for an answer and she finally blurts out the truth--that Murray's daughter found them in her father's bedroom--Gould blanches.

But from here it's a steady decline into tears, cliche's, gratuitous smooching and a lecture at the end. Midway, Michael stalks his unfaithful wife and friend, walks in during a moment of afternoon delight and shouts at them, "You have no idea how ridiculous you look. 'The Fornicators! An El Cheapo Porno Production!' " "The Rules of Marriage" is by no means pornography. Just another case of network television pandering.