"Young Adult" is a rare, genuinely original comedy. Director Jason Reitman re-teams with screenwriter Diablo Cody — that's the duo that gave us "Juno" — for a sardonic look at a former high school bombshell wallowing in the depths of delusion.

Mavis Gary (Charlize Theron) was the hottest girl in school. The character is a perfect stand-in for all those unattainable, popular girls whom other girls envied and hated and all the guys wanted to be with. She probably won homecoming queen, prom queen, most likely to succeed and all the rest of it.

Post-high school life began to pan out as expected for Mavis, too. She married and found a steady gig as a writer for a young adult book series reminiscent of "Sweet Valley High."

By the time we meet Mavis, though, she is a wreck. She is divorced. The book series, which is painfully awful to begin with and never launched her beyond the disheartening anonymity of a ghost writer, is about to end.

Mavis is an alcoholic. She sleeps with men she doesn't care for or even remember the morning after.

Mavis is hitting bottom. Everyone can see it, except her.

None of this sounds funny on the surface, but Cody, Reitman and Theron have made it so. "Young Adult" pushes the envelope of sarcastic humor without becoming parody or caricature. Mavis is such a mess that it's funny, and the filmmakers found ways to let the audience in on the joke rather than making the whole thing so sad or bitter it turns us off.

See, another part of Mavis' downward spiral is a staggering degree of delusion.

She receives an email announcing that her high school flame Buddy (Patrick Wilson) and his wife Beth (Elizabeth Reaser) have had a child. Mavis somehow interprets this email as a cry for help from Buddy. They haven't spoken for years, so her inclusion in this mass email surely signifies Buddy's longing for her, right? Right.

Mavis heads back to her small Minnesota hometown with a scheme to win Buddy back - or in her mind, to rescue him.

Theron is the ideal actor to play Mavis. She demonstrated her ability to transform into grotesque characters by playing a strikingly human serial killer in "Monster" (2003).

She doesn't get nearly as ugly here, but she takes on the disaster that is Mavis with abandon. She sleeps sprawled out, face down, still wearing last night's clothes. The key to her morning routine is chugging a two-liter of soda. Her standard daytime attire consists of sweatpants and worn-out T-shirts.

However, we watch Mavis go through a metamorphosis each time she prepares to meet Buddy, but it's a painful metamorphosis. Recapturing her former luster requires a mani-pedi, various scrubs, a trip to the mall and an extensive cosmetic regime.

The end result of all this is that Mavis looks like, well, Charlize Theron. No amount of external beauty, however, can hide Mavis' desperation and madness.

Part of the joy is that Mavis doesn't even try to hide her intentions. Her plan is ridiculous, immoral and doomed to fail, but she just goes for it.

Equally joyous is Mavis' sparring with Matt (Patton Oswalt), a former classmate with permanently disabled legs and razor-sharp wit. He warns Mavis against following through with her plan, even as the two form an odd bond, based initially around Matt's homemade whiskey distillery.

Matt's younger sister Sandra (Collette Wolfe) is also a key character. Sandra still worships Mavis like a beauty queen. She is stuck in arrested development even more than Mavis.

"Young Adult" is consistently surprising. It stretches out the central joke without pushing it too far, and it does not end the way we expect. Whether in sweats or a little black dress, Mavis is a train wreck we can't look away from.

Jeff Marker teaches film and literature at Gainesville State College. His reviews appear weekly in Get Out and on gainesvilletimes.com/getout.