IN "ABOUT Schmidt," a 35-foot motor home waits silently outside the Nebraska home of insurance executive Warren Schmidt (Jack Nicholson) and his wife, Helen (June Squibb). The Schmidts intend to drive it across America, as soon as Warren's retirement takes place.

But when Warren finally leaves the Woodmen of the World Insurance Co., there's a problem. Helen passes away unexpectedly. Warren has already begun to realize his spiritual tank is running dry, especially after a particularly dehumanizing retirement dinner, whose most compelling symbol is a giant cake made to resemble Warren's glass tower building.

Warren decides to make the trip anyway. His destination: Denver, where his estranged daughter, Jeannie (Hope Davis), is about to marry Randall (Dermot Mulroney), a water-bed salesman who promises Jeannie a life of further emptiness.

Warren's genuinely appalled at Randall, a well-meaning buffoon with a mullet and a rather ridiculous goatee. And he's not too thrilled with Randall's mother, Roberta (a gutsy, memorable Kathy Bates), a kook with a powerful sexual appetite.

Warren sets about trying to save his daughter from a future of American-issue mediocrity.

"You have an opinion -- now?" Jeannie asks her father.

Although the story moves as slowly and flatly as its Midwestern setting, there are powerful rumblings at work just beneath the surface. Warren is learning about the emptiness of his life and what he can -- and cannot -- do about it.

The seeds of a transformation are set before the trip, when Warren is channel surfing in his lonely home. When he chances upon one of those save-the-African-children infomercials, he signs up to sponsor a Tanzanian child at $22 a month. Confirmation of his membership comes in the mail, including a photo of the boy he's supporting.

Warren begins writing to 6-year-old Ndugu, his new child. And in so doing, Warren begins to assess his life for what it is, and isn't. There is amusement in these letters, which he writes on the road. Warren writes on a level that Ndugu couldn't possibly understand, as he decries his marriage or recites, with actuarial aplomb, the statistical odds of his own demise.

"Life is short, Ndugu," he writes. "And I can't afford to waste another minute."

Once again, co-writer/director Alexander Payne and co-writer Jim Taylor, who made "Citizen Ruth" and "Election," have taken an American canvas and turned it on its side. But they stop refreshingly short of postmodern ridicule. Certainly, the movie enjoys a series of gentle smirks at middle America, but it does so with the satirical equivalent of a big hug. It's as if Samuel Beckett collaborated on a drama about American retirement with Preston Sturges, while Larry ("Curb Your Enthusiasm") David hovered somewhere in the background.

"About Schmidt," the composite of an early Taylor screenplay and extremely loose borrowings from the novel of the same name by Louis Begley, is a strange but touching hybrid. It's a comedy that only hints at punch lines, that likes to show the seamless connection between tragedy and comedy, and that never loses track of its central character's subtle changes.

What gives "About Schmidt" its ultimate boost, what pushes it into the stirring heavens is Nicholson, who produces the most understated -- and one of the most powerful -- performances of his career. The actor refuses to give the audience that sidelong wink, that reassurance that we're embarking on another Jack jaunt. This movie is about Nicholson playing against himself and all that we expect about him. And it's the test of a great actor that he loses none of our enrapt attention, despite playing a man whose face is a frozen mask of concealed resentment and bitterness. And when that mask cracks, the change is so profound, our hearts can only follow the same course.

ABOUT SCHMIDT (R, 125 minutes) -- Contains obscenity and nudity. At Loews Georgetown.

Jack Nicholson plays a retiring insurance salesman in "About Schmidt."