Smart, sexy, funny and poignant, “Up in the Air” makes it three for three for Jason Reitman, following “Thank You for Smoking” and “Juno.” This adaptation of Walter Kirn’s novel is breezy and effervescent, but it has a serious undertone (accentuated by its concern with the current economic downturn, with its increasing rate of unemployment), and it’s easily one of the year’s best films.

It’s basically a story about commitment—or refusal to commit—but that hoary cliché is treated here with the sophistication characteristic of classic Hollywood comedies of the thirties and forties rather than the sophomoric puerility that most modern-day studio pictures ladle onto it. George Clooney, again showing that he’s the modern equivalent of Cary Grant, plays Ryan Bingham, a fellow with an Omaha-based firm that specializes in firing people whose companies want to give them the axe but don’t have the guts to do the deed themselves. A rootless guy whose relationship even with his own sisters is strained and who moonlights giving “inspirational” speeches about freeing oneself from encumbrances both material and personal, Ryan rejoices in spending most of his time traveling from city to city to do his job. He keeps a sparsely-appointed apartment in Nebraska, but is almost never there, preferring his hotels, air terminals, and the blissful hours in transit, which he hopes to multiply in order to win a rare American Airlines card that recognizes a huge number of miles traversed. In these places he’s treated as somebody special, and revels in it.

Ryan’s routine is threatened, however, when his hard-hearted boss (Jason Bateman), who sees the recession as a boon to his business, aims to cut expenses by following a scheme proposed by perkily confident company newcomer Natalie Keener (Ann Kendrick) to keep the front-line fire-men at home and terminate folk via computer connections. Ryan protests but is forced to go out on the road with Keener to show her the ropes so that she can understand the realities of the process and perfect her “script” accordingly. Much of the picture is of course devoted to the banter as this unlikely pair get to know one another and confront the actuality of dismissing workers who suddenly must face the fact that their livelihood is completely unraveling. The montages Reitman and Sheldon Turner have fashioned from these encounters, some fairly extended (as with J.K. Simmons and Zach Galifianakis) but most fairly brief, add a topical punch to the material.

But that’s only one layer to the film. While traipsing around with Natalie, Bingham is conducting an easygoing affair of sorts with Alex Foran (Vera Farmiga), another airline roadie whom he’s just met, had hot sex with, and is now arranging trysts with whenever their schedules allow—a relationship with absolutely no long-term ties on either side. Alex seems Ryan’s perfect partner—one who shares his love of being constantly on the go but expects and wants nothing more than the occasional evening together. Of course Bingham becomes more serious about her than that—something that Keener encourages even as her own Omaha beau is breaking up with her via text message (a fact that gives Clooney one of his best lines)—and winds up inviting her to accompany him to the Wisconsin wedding of his younger sister Julie (Melanie Lynskey). There he acts a completely foreign role when Julie’s intended Jim (Danny McBride) abruptly gets cold feet and he must step in to argue the value of marriage. It gets him thinking about what he really wants; but the picture throws a curve—a series of them, actually—that end things on a bittersweet note that casts a pall even on Ryan’s achievement of his long-held mileage dream.

The picture is kept aloft by witty writing and Reitman’s adroit direction, and by Clooney’s effortlessly charismatic turn, but the rest of the cast is great as well. Farmiga proves an equal partner in every respect, bringing the same sort of spark Hepburn brought to her pairing with Tracy, and Kendrick endows Natalie, who could easily have become simply an annoying twit, with some genuine gravity of her own. Bateman is underused—one can imagine much more having been done with his character—but Lynskey, and especially McBride, prove real scene-stealers, as do Simmons and Galifianakis, while Amy Morton practically channels Allison Janney as Ryan’s older sister, who’s having relationship problems of her own. And an amusingly mustached Sam Elliott adds a bright cameo as a pilot. Add colorful production design by Steve Saklad and art direction by Andrew Max Cahn, enhanced by crisp cinematography by Eric Steelberg, and you have a very attractive package. Rolfe Kent’s jaunty score adds to the enjoyment; and be sure to stay through the song that accompanies the closing credits.

“Up in the Air” is a film that defies current comedic conventions. It’s not gross—not a crotch-punch in sight, no nasty language, nary a sign of potty humor. Maybe that will doom it with some audiences. But it flies high on the old-fashioned virtues of intelligence, wit, and cleverness—with a dollop of seriousness added to the mix that recalls the combination the great Preston Sturges brought to a picture like “Sullivan’s Travels.” This is the rare movie about the fear of commitment to which you should definitely not be afraid to commit.