Described as a “psychotic prom-queen bitch,” the antiheroine of Young Adult is a prize part that affords Charlize Theron one of the season's prize performances. Theron established her bona fides and won an Oscar playing a serial killer in Monster; Young Adult, directed by Jason Reitman from Diablo Cody's screenplay, allows the glamorous actress to take on another traditionally “male” role. Here, she's something like the early-'80s Bill Murray: a slovenly, spiteful, self-absorbed, snobbish, smart-mouthed case of arrested development — the precise opposite of the maddeningly precocious little protagonist who gave her name to Reitman and Cody's previous collaboration, Juno.

Indeed, unlovable Mavis is pretty much the opposite of Juno's egregious cherub — a 37-year-old, Minneapolis-based writer (or ghostwriter) of high school romance fiction, as well as a divorcée and an alcoholic.

While working on the last book in the series that has served as her sinecure as well as an alternate reality, she is plunged into an existential crisis — if not temporary insanity — with the receipt of a mass email announcing her high school boyfriend Buddy's fatherhood. Leaving the previous night's date snoring in her bed, she packs up and drives back to Mercury, Minn., while playing a vintage mixtape heavy on the Replacements.

A woman on a mission to break up the boyfriend's marriage and — who knows the precise fantasy? — maybe start anew, Mavis cold-calls her ex (Patrick Wilson), then heads to a sluggish bar inhabited by an affable schlub (Patton Oswalt) who instantly recognizes her from high school and whom, after a few drinks have blunted the edge of her hauteur, she remembers as “the hate-crime guy” — a loser mutilated for life by a gang of jocks who imagined he was gay.

Among other things, they now bond in their distaste for Mavis' cousin — “the popular cripple” whose automobile accident, Mavis complains, “totally ruined” her Sweet 16 party — as well as their shared adoration … for Mavis. Not for the last time does this mismatched pair of adolescent antipodes get hammered together.

Mavis writes about high school and, in essence, she still lives there — eavesdropping on mall girls and incorporating their patois into her books and, when needed, patois.

An extended lost weekend with intimations of Groundhog Day, Young Adult has the former prom queen waking each morning from a drunken stupor to spend the afternoon dolling herself up for the evening's “date” with good-looking dumb bunny Buddy. These increasingly disastrous tête-à-têtes range from a drink at the local sports bar to a barroom performance by Nipple Confusion, the all-femme amateur band in which Buddy's good-natured wife (Elizabeth Reaser) bangs the drums — and to which Mavis shows up looking like Olivia Newton-John in the final scene of Grease — to the new baby's naming ceremony, a ritual seemingly attended by the whole town of Mercury, including Mavis' parents.

Young Adult might be brushed off as curdled romcom were it not for two things: The first is the depth of Theron's performance. As outrageously mean, dissolute, bratty and delusional as Mavis becomes, she never ceases to inspire sympathy — even when she demands it.

The second, less predictable aspect is the utter absence of the corny rehabilitation found in Juno and Reitman's glib downsizing dramedy Up in the Air. Mavis might hit bottom (or not), but there's no pot of small-town virtue at the end of this narrative rainbow. For all the revelations regarding the roots of her Mavis-ness, the girl's bitchiness remains intact. She is forever young.

YOUNG ADULT | Directed by JASON REITMAN | Written by DIABLO CODY | Paramount Pictures | ArcLight Hollywood, Landmark

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