Charlize Theron has never let her beauty get in the way of her art. In "North Country" she was a smudge-faced iron miner and single mother. In "Monster" she looked like the wrath of god as a foul-mouthed prostitute and serial killer, and won an Oscar for her efforts. "Young Adult" is another story, though. The movie itself, directed by Jason Reitman from an original screenplay by Diablo Cody, is a cockeyed comic triumph that flashes between bright and dark like a strobe light of the spirit. And Ms. Theron, as Mavis Gary, a self-styled author rather than a mere writer, succeeds sensationally at something much harder than playing ravaged. Mavis was once prom-queen beautiful; these days, in her late 30s, she can still pull herself together to fairly dazzling effect. But she is also sly, selfish, nasty, hurt and hurtful, dryly sardonic and fatefully deluded. What makes her such a marvelous character is what seethes deep beneath her skin.

What makes the movie marvelous is the same combination that the filmmaker and writer brought to "Juno"—unerringly subtle yet precise direction plus a literate script with dramatic energy and a delicate tone. (Mr. Reitman directed and co-wrote "Up in the Air" as well.) The movie's title refers to the genre of books Mavis authors, or more properly ghost-writes, and to Mavis herself, the suggestion being that she is young for her age. But young can't begin to describe her immaturity, or her failure to flourish as an adult. When we first see her in Minneapolis, a slattern in a junk-strewn apartment, she's living on scraps of loveless sex, drinking too much and picking up snatches of kids' conversations here and there, then dropping them into her chronicles of Kendall Strickland, the teenage heroine of a series called Waverly Prep that is, in fact, headed for the dustbin of subliterary history.

Suddenly, though, the blockage in Mavis's life gives way to a bizarre breakthrough, and she becomes a person possessed by a mission—reclaiming the love she lost when her high-school sweetheart got married. (He's played by Patrick Wilson.) To find her old flame, she goes off to her hometown in the company of her better half, a yappy canine pipsqueak named Dolce. As she drives, she listens to old tapes on the car's cassette player while we see the player's innards whirring and spinning in macroscopic detail; outdated technology as an access route to the listener's past, and an emblem of her stunted psyche.

It's a clever flourish in a film with a distinctive look (Eric Steelberg was the cinematographer, and Kevin Thompson designed the production) and an exceptional cast. Mr. Wilson's Buddy, quite happily married with a new baby, can't fathom what Mavis is up to—his puzzled decency is painfully charming—but she's as transparent as a wine-stained carafe to an old classmate named Matt Freehauf, who is played to laconic perfection by Patton Oswalt.

"Guys like me were born loving women like you," Matt tells her ruefully. They are two of a kind with a kinship in misfortune, much of it self-inflicted: Matt has used the small town as a refuge since high school, when a misconstrued hate crime left him significantly but far from seriously disabled. They are also a classic comic pair in a production that's chockablock with vivid characters: Elizabeth Reaser as Buddy's wife, Beth; Collette Wolfe as Matt's wife, Sandra; Louisa Krause as a droll motel desk clerk.

Still, the movie belongs to its star at every moment she's on screen. This is partly a function of Ms. Cody's script, which depends on a daring supposition—that audiences will invest their attention in a heroine who, from the start, confronts life with a toxic vibe. (I think they will, once word gets out that Mavis is as funny and surprising as she is.) It's equally a tribute to Ms. Theron's fearless reach and formidable range in a role that takes her from wry comedy into something perilously close to an American tragedy. By the end, and against strong odds, "Young Adult" gives us a grown woman to weep for.