There”s a single line-make that a single word-in the opening reel of “Young Adult” delivered with such pointed lack of empathy as to immediate wipe clean any cosier expectations we might have had of a second collaboration between director Jason Reitman and screenwriter Diablo Cody. Staring disconnectedly into her glass while on a blind date with a seemingly decent chap wittering on about his experience of teaching in South East Asia, Charlize Theron”s divorced, 37 year-old youth fiction novelist Mavis Gary screws up her face and spits out the question, “Why?”

The guy doesn”t acknowledge the question; indeed, it doesn”t break his flow for a second. But after Theron”s drolly apathetic tone gets the required laugh from the viewer, her sourly confused expression seals the moment as more than a snarky throwaway: this isn”t just a woman who disdains people who help others, it”s one who sincerely doesn”t comprehend them. A kind of high-functioning autism invisible beneath her snippy intelligence and immaculate lipstick, Mavis”s misanthropy makes in her mind a gigantic ‘why” of all human relationships, though she”s sufficiently self-possessed enough not to care about the answers. We never see the face of her hapless date in that early exchange; in a sense, one doubts she does either.

If not quite a brilliant film itself, “Young Adult” is nonetheless shot through with such brilliant flashes of testy insight: it”s certainly as daringly ungenerous a mainstream comedy as American cinema has served up in recent memory, somehow standing on both sides of the mirror as it initially ridicules both Mavis for her unearned snobbery and the white-bread Midwestern society around her for allowing itself to be ridiculed. Like the pithily condescending character comedies of Alexander Payne, Cody”s script places immense stock in social and geographical limitations: Mavis assumes the classic American character mantle of the small-town-girl-made-good, returning home from the big city, but the film”s greatest unarticulated joke is that big-city life in this case lies only as far afield as Minneapolis.

In a series of tersely edited, beigely art-directed scenes at the outset, Reitman swiftly establishes the contained sense of failure surrounding the former high-school prom queen”s supposed urban self-realization: living in a boxy new-build high-rise with only Starbucks-flavored friends and a virtually clockwork Pomeranian for company, ghost-writing Sweet Valley High clones using dialogue cribbed from unwitting local teens and routinely passing out on unmade beds, Mavis”s city life seems so scaled-down, unambitiously tethered to lifelong bad habits, that it”s no surprise when, at a supremely loose end post-divorce, she returns to the smaller but similarly sterile pond of her hometown.

The ostensible, wholly deluded goal of this trip may be to win back Buddy, the jockish high-school sweetheart (Patrick Wilson) who gave Mavis her few isolated years of emotional security-that he”s blissfully married with a newborn child is but a minor irritation to her-but while the film plays the spiky farce of this scenario for good, nasty laughs, the moving, more complicated subtext is that she”s competitively chasing a life she knows she doesn”t want for herself.

Theron is superb at registering Mavis”s split-second shifts in awareness with a single facial movement, and there”s a key one when she arrives, ludicrously overdressed, at the synthetic sports bar where Buddy has scheduled a friendly reunion: the fall in her face says everything as she realizes the ex she”s held up all her adult life as the tantalizing road not taken is not just unavailable, but a dullard to boot.

Wickedly funny as it so often is-Cody”s voice has hardened and settled since her Oscar-winning work on “Juno,” now choosing its moments to be strident or smartass-“Young Adult” works best as a tricky small-town tragedy in which everyone wants someone else”s life, but no one”s life looks particularly desirable in the first place. A safer film (Reitman”s own “Up in the Air,” for example) might cast its moral lot on one side of the net, idealizing either warm domesticity or no-strings irresponsibility, but “Young Adult” is too wary and too searching for that.

Buddy”s backyard-barbecue lifestyle looks no more appropriate a solution to Mavis”s malaise than that of her disabled former classmate and unlikely kindred spirit Matt, played with wry, whisky-marinated warmth by Patton Oswalt in full-schlub mode. Ingrained codes of Hollywood romance tell us her redemption lies in a choice between them, but the film”s courageous, potentially divisive third act suggests that self-awareness, once attained, needn”t be accepted.

This is a bold narrative endgame, but Cody”s script makes some specious judgments to get there: her even-handedness is commendable, but she risks patronizing everyone in the film to exhausting effect, notably in a climactic bar-seduction scene where Mavis”s would-be cosmopolitan sluttiness and the low-rent naïvete of Buddy”s wife”s dopey garage band are pitted against each other to equally cruel effect. There”s a tonal thinness to such stretches where, far less sophisticated construction that it was, some of the open-hearted humanism of “Juno” wouldn”t go amiss.

Happily, these rare instances of undue meanness in the writing are elegantly papered over by Theron”s ferocious star turn: her sharpest, most fluidly capricious work since her career-shifting performance in “Monster” eight years ago, and a welcome workout for the vinegary comic timing that”s been curiously underused since a few far less worthy early-career vehicles. (It”s also arguably the first role of the actress”s career to make interesting, counter-thematic use of her traffic-stopping beauty.)

It”s to Theron”s considerable credit that, as inscrutably unlikeable as Mavis remains for nearly three-quarters of the film”s running time, we”re never really off her side: even in her worst moments, there”s a kind of purposeful poise to her brazenly atrocious behaviour that”s inexplicably admirable. It”s easy to imagine an actress like Katherine Heigl humiliating Mavis by playing down to her; it”s Theron”s sympathy for the devil that makes this pleasingly peppery, savagely sad character study fly. Even to the ugly end, not everyone sees through her: when a sweet loner confesses to Mavis how much she covets her life, it”s the audience”s turn to bewilderedly ask, “Why?” 

For more views on movies, awards season and other pursuits, follow @GuyLodge on Twitter.

Sign up for Instant Alerts from In Contention!