Paul Newman is no longer the face on the sauce bottle. As weather-beaten private investigator Harry Ross, his expression is tight, military, unamused, the voice bruised and grainy. Robert Benton's Twilight begins with Ross sweaty and gasping beer at a Mexican poolside, while trailing a pair of runaway lovers. It's the archetypal LA gumshoe scenario, but you never saw a gumshoe who looked so unwilling to be part of it, as if he'd played the part a few times too many.

Scripted by the novelist Richard Russo - who last worked with Benton on Nobody's Fool, another Paul Newman vehicle - Twilight is a return to the familiar haunts of LA noir, in many ways unchanged since Philip Marlowe plied his beat. It reworks the commonplaces of the private dick genre, but instead of playing them as cliches - as arch flickers to relish and mock in the same breath - it lets them resound as bitter, melancholy echoes.

Most of the echoes come directly from Raymond Chandler territory. There's a rendezvous with one Gloria Lamar, and a stooge with the impeccably Chandlerian name of Lester Ivar, who figures in the story mainly so that the magisterially unwieldy character actor M Emmet Walsh can appear just long enough to shatter a shower window in his death throes.

Twilight's tarnished, everyday 1990s Los Angeles is still haunted by Chandler's ghosts. Everyone still has their skeletons, and Marlowesque PIs still set out to unearth them at all costs; yet no one seems too bothered by what's likely to turn up. Of course there's a body waiting to be found, but whose it is, and how it got there, is less important than what it tells you about how people manage to live with their secrets.

Twilight is less a straight cop story than a disquisition on fading. The characters, the city, the genre are all at the end of their respective roads, but struggling on anyhow. Harry, played by Newman with a tender attention to the weight of dreary years and disappointment, is a study in dignity against the odds. Invalided out of the PI business, he's living as an indulged retainer at the house of Hollywood potentates Catherine and Jack Ames (Susan Sarandon and Gene Hackman). Jack, diagnosed with cancer, sends him on one last mission - which turns out to be a kind of symbolic final reckoning with the shamus genre and its cliches.

For a while, the familiarity of the imagery - like a gun nozzle peeking out of the darkness - engenders a sort of reassuring cosiness. So do the stars, class acts all, laden with echoes of earlier movies. Hackman, as the kingpin laid low, is more weary and dough-faced than we've seen him before. Sarandon, as an autumnal screen goddess, is snappy, snake-eyed, acerbic. Stockard Channing works her sexy, quizzical creases as Ross's flirty cop adversary, Verna. The cherry on the cake is James Garner, droll and shambling, baggier than in his Rockford days, and making rueful remarks about prostate trouble.

Perhaps it doesn't seem right that a thriller should impart such jovial well-being - and, sure enough, Twilight sets us up for a fall. The cosiness has hard regret as its flipside. Like all noir, Twilight is haunted by the prospect of impending death: but here it's less the sudden gun in the dark than the creeping mortality written in people's faces.

Again like much great noir, Twilight casts a cold eye on social structures. Harry is the trusted insider, but when the chips are down, he realises the Ames' sulky, neglected daughter (Reece Witherspoon) is right - he is just the hired hand. There are the golden people and then there are the gumshoes - the LA underclass who clean up the mess. As Garner's character says, "Don't you get sick of the beautiful people?" In a further, tender twist, the beautiful people turn out at the end to be motivated by an unfathomably tough unconditional love.

Twilight is shot by Piotr Sobocinski with an infallible eye not just for atmosphere but for architecture, too - the film is an eloquent essay on west coast ranch modernism. It may be a prime example of minor mainstream cinema at its best, but Twilight's concern with maturity makes it a rather radical gesture in youth-fixated Hollywood - no wonder it bombed in the States. Not everything gels: Giancarlo Esposito's Hispanic sidekick is awkward comic relief, as bogus a genre throwback as Charlie Chan's number one son. But in a quiet way, Twilight is more daring a reworking of noir than the glib bravado of Steven Soderbergh's new Elmore Leonard adaptation, Out of Sight. "You'd think that . . . the world would lose its power to seduce, but you'd be wrong," says Harry early on. He means the eternal appeal of LA, and of the private-eye genre, and he proves his point.

"Twilight" (15) opens nationwide on 4 December