Attention anyone who has ever complained about the lack of movies for adults and about adults: Now is your chance to prove to the studio suits a great film about growing old is economically viable.

"About Schmidt" is an unaffected, quietly ingenious, wonderfully melodious, melancholy comedy starring Jack Nicholson in what may be the most tactile, nuanced and natural performance of his exalted career. He plays Warren Schmidt, a former insurance actuary from Omaha, Neb., whose life has become untethered in the double-whammy wake of recent retirement and sudden widowhood.

Trying to cope with a glut of old emotional baggage, Schmidt sets out on a soul-searching trip -- the eventual destination of which his daughter's unfortunate wedding to a mullet-headed waterbed salesman -- in the monstrous, 35-foot deluxe motor home he'd reluctantly purchased at his loving late wife's behest.

He drives to the small town where he was born, hoping to find some solace but instead discovering a tire store where his childhood home once stood. He visits the University of Kansas, where he's met with polite indifference by the boys at his old fraternity, whom he'd naively hoped to regale with tales of his college days. He learns the dos -- and especially the don'ts -- of RV park camaraderie when a younger couple with a similar rig invite him to a dinner that goes badly as Schmidt's loneliness rears forth unexpectedly.

And the whole journey (both literal and metaphysical) is documented in hilariously frank, compulsively long letters Schmidt writes to "little Ndugu," a Tanzanian orphan he sponsors for $22 a month through a charity he saw advertised during a depressed bout with late-night TV addiction.

For the first time in probably 20 years, Nicholson as we know him vanishes into this performance -- and it not because of his extreme comb-over, his generic blue suits, or the way he walks with a bit of a stiff waddle, his hands always clenched into loose fists. It's because the actor ingrains Warren Schmidt with an acute, unvarnished, regret-burdened ordinary-ness.

From his first scene in the movie -- a shot from above in his now-emptied office as he waits with a bemusedly heavy heart for 5 p.m. on the day of his retirement -- to the sincerely sentimental, hard-earned later epiphany that answers Schmidt's questions about his own value in life, Nicholson hasn't a single familiar mannerism or false moment.

Directed by the subtly eccentric and imaginative Alexander Payne (who also co-wrote with his "Election" and "Citizen Ruth" collaborator Jim Taylor), "About Schmidt" is slice-of-life filmmaking at its sharpest and truest. It's wry but it's genuinely poignant. It does not turn middle age into a source of "frisky" old people punchlines. And it warmly embraces not only the personality of the Midwest (no Hollywood hayseed caricatures here, thank you very much), but the atmosphere as well.

Payne has a penchant for funny, life-like details (a bug splattering Schmidt's clean windshield on the freeway causes a frustrated pause in his narration) and for capturing the essence of real locations (no soundstages here, thank you very much). Sometimes it's an overcast Interstate surrounded by farms (you can almost smell the cold, post-rain humidity) and sometimes it's the house Schmidt has lived in for 40 years, with its decades-old vacuum cleaner, its wind-up alarm clocks and the free-standing automatic dishwasher in the otherwise outmoded kitchen.

When Schmidt gets to Denver for the wedding, the movie becomes a little lighter, even though our antagonized protagonist is more out of sorts than ever, struggling as he is with his desire to talk his discontented daughter Jeannie (Hope Davis) out of her engagement. Should he risk alienating their already rocky relationship and compounding his emotional isolation? Or should he keep his trap shut, as he's been wont to do his whole life?

It doesn't help that his torpid future son-in-law Randall (Dermot Mulroney) is soliciting him for an investment in a pyramid scheme, or that the dope's aging hippie mother (Kathy Bates in a sublimely kooky performance) makes a pass at Schmidt in her hot tub while he's trying to soak his muscles after a bad night on the cheap waterbed in Randall's childhood bedroom. (Another whimsical Payne touch: the bedroom walls are decorated with framed "certificates of completion" for two-week courses and blue ribbons for "participation.")

"About Schmidt" is never more steeped in its themes of consternation and internal conflict than at the wedding, where Nicholson's impeccable performance comes to a pinnacle as that dam holding back Schmidt's compounding grief threatens to crack.

But this isn't a tale of sorrow so much as a droll yet heartfelt, remarkably human and uniquely American magnum opus of sorrow-induced psychological self-examination. And there is a light at the end of Schmidt's tunnel as profoundly affecting as it is sweetly, surprisingly simple.