For the longest time in "Home Front," the family drama starring Carroll O'Connor that opened last night at the Royale Theatre, nobody mentions Vietnam. The topic is verboten in the Collier household, even though the Colliers' 23-year-old son has returned from "over there," clearly traumatized by the experience, and has taken to sitting on the back porch and staring broodingly into space.

It is Thanksgiving Eve 1973 -- and then Thanksgiving Day -- in a comfy suburban bungalow in the Dallas-Fort Worth area. Any talk of Vietnam would clearly mar the holiday and imperil the sense of togetherness that tradition demands. So the other family members -- the mother, a wound-up flibbertigibbet (Frances Sternhagen); the father, seemingly a pillar of good-natured patience (O'Connor); and the daughter, a pretty party girl verging on petulant (Linda Cook) -- all try to pretend the problem doesn't exist.

Jeremy (Christopher Fields), as the disturbed vet is called, is just in a "bad mood," feeling sorry for himself.

In this, his first work for Broadway, and one of the few serious American plays to open this season, writer James Duff is chronicling with a good deal of accuracy and humor the everyday ways of avoidance. The Colliers are not only sidestepping the unpleasantness that was Vietnam; they are dodging one another. Legitimate as the theme may be, however, it comes with a built-in trap. And Duff, I fear, has tumbled into it. His characters are so adept -- and so persistent -- at hewing to trivialities that when Jeremy finally cracks up, the explosion seems to spring from a different play entirely.

For three of its four scenes, you see, "Home Front" behaves like a sitcom, in which bickering is the essential order of business. Who dipped into the homemade peanut brittle before the appointed hour? Does Thanksgiving Day breakfast have to be at 8 a.m. sharp? Why won't Jeremy play a few hymns at the piano to set the festive mood? And when is he going to change out of his jeans?

If the daughter happens to mention that Jeremy was screaming in his sleep the night before, the observation is granted no more importance in this household than, say, the mother's reflections on Ingrid Bergman. ("I don't like the way she lives," snaps Sternhagen crisply. "She started a trend, is what she did.") Then, in its final quarter hour, "Home Front" lurches into melodramatic hysterics. Vindictiveness comes pouring out of the characters' mouths, along with some long-buried secrets. A gun is brandished wildly and while it never goes off, a couple of symbolic murders are nonetheless perpetrated before the final curtain.

Without warning -- or at least fair dramatic warning -- Duff has hurtled us into another emotional country altogether. It may be the true habitat of this repressed clan, but the climax seems less the logical extension of what's gone before than a calculated flip-flop. First staged in England last summer, "Home Front" (then called "The War at Home") was to have been directed by Alan Schneider, who was struck and killed by a motorcycle just before rehearsals began. Whether he could have provided the dramatic foreshadowing and darker tones that would prepare the play for its harrowing conclusion is open to conjecture.

The staging by his replacement, Michael Attenborough, goes in precisely the opposite direction, however. It hypes the comedy, further unbalancing the dramatic equation, and turns Sternhagen, that splendid character actress, into a shrill caricature. Not since Arnold Beckoff's mum came to call in "Torch Song Trilogy" has Broadway seen quite so exasperating an example of meddlesome motherhood. Sternhagen's character doesn't even realize she's carping and nagging; in her mind screaming is simply "using my loud voice" and arguing is "thinking out loud." Funny and crowd pleasing as the performance is, it is also increasingly unreal.

Jeremy, taciturn and moody for most of the play, should be a time bomb, ticking away. But as Fields portrays him, he is no more threatening than an alarm clock and when he finally gives vent to his nihilistic agony, it registers as an excessive temper tantrum. Cook, meanwhile, plays the vapid daughter, as if she were auditioning for summer stock.

The one surprise of the production is O'Connor, as the easy-going father who would prefer to dwell peacefully in his armchair doing crossword puzzles. What O'Connor does with a quiet choke in the voice, an ironic twinkle in his eye or a tiny hitch in his delivery is enough to banish any memories of blustering Archie Bunker. The performance is beautifully modulated; it alone makes a successful transition to the play's heated climax.

To O'Connor falls the curtain line. Surveying the emotional wreckage of the holiday, he announces, "We're going to forget it ever happened . . . It's the only thing we can do." As he speaks, his ripe jowls tighten, the rosy color drains from his face, and his voice assumes the firm edge of the law-maker. Jeremy is no more. Vietnam never was. And reality will be tucked away neatly in the closet next to the cardboard Pilgrim figures that have decorated the mantle.

On the home front, empty cheer reigns triumphant.