Mavis Gary is a fragile tree waiting for a windstorm to bring her down. Mavis Gary, a slim, 37-year-old aging prom queen, is a ghost writer for teen-lit books. Thousands read her. No one sees her. She’s still much the same person she was in high school, the prom queen but with no king, the queen bee of the mean girls sans entourage. She lives alone high above the madding crowd. But she’s gorgeous, talented, desirable. What’s wrong with this picture?

Mavis, of course, is the amazing Charlize Theron, and her character, beautifully etched, was conceived and written by the equally amazing Diablo Cody, who gave us the great “Juno.” Both women will certainly catch their high heels on the red carpet come spring. I love when that happens.

In the beginning, there is darkness upon the earth Mavis inhabits. Each day the sun comes up on her like a migraine. Her life is a wall calendar of bad nights and cluttered days. Mavis drinks. A lot.

Payback is some years off, and while she still looks like a college senior, her fault lines are widening and the seismic tremors are coming. Even well paid, she lives on junk food and quarts of sugary Coke, the manna and holy water of the alcoholic. Mavis has come to a dead zone.

Cody and the talented director Jason Reitman set Mavis up for us right away. Her apartment, an expensive high rise in Minneapolis, is a mess. She has poop pads for her little dog, and then sets him and his tiny food trays out on the patio where five or six empties sit in the sun. She dresses in last year’s college casual, and works hard to keep her eyes open long enough to finish her book which is the last in the series. The future is a blank.

One foggy morning, her world is rocked by a Facebook note about her ex high school lover (Patrick Wilson) back in sleepy Mercury, Minn., “Fargo” with higher SATs. He and his wife have just had a baby. Mavis snaps. All of this, she thinks, could have been hers, him, the vine-covered cottage, babies and bacon on the grill. She concocts a plan. She will go back to Mercury, snatch her old passion from his wife and happy home, and start anew with him by her side.

Reitman presents Mercury in a wonderful all-the-same American tapestry. Slowly, Mavis, in pursuit of her lost dream, drives past a chilling montage of 2011 mid-American culture: a Staples, Chillis, Burger King, WalMart, Home Depot and Dunkin Donut landscape.

Her first stop, of course, is a bar where she inhales several Maker’s Mark bourbons, and meets up with an old classmate she doesn’t remember. “Your locker was right next to mine,” he says, “You never spoke to me. Just your mirror.”

This is Matt (comedian Patton Oswalt) who, in high school, was beaten and sexually crippled by high school jocks who thought him gay. Cody has given us two crippled pilgrims, lost in the dark nights of their souls, whom fate has connected. We hold out hope that fate has a plan in mind.

Cody has clearly set Matt up here to become Mavis’s conscience, her cynical Jimminy Cricket with a cane. After many drinks, when she drunkenly outlines her elaborate plan to seduce her old school beau away from his meaningless, hum-drum existence, Matt, a clear-eyed realist, sees the disaster coming. This is his town and despite his own pain, he loves it and all in it. He knows that the old boyfriend is a happy man who loves his wife, and he sees Mavis as a run-away train about to leave the tracks.

Reitman and Cody have put together a jewel of a movie. Cody’s citizens of Mercury are deliciously cast, evoking that wonderful mid-American mixture of the serene and the bland, the boring and contentment of small town everywhere. Wilson as the old boyfriend, and Elizabeth Reaser as his wife, are picture perfect, as are all the players.

Reiser is wonderful as the wife who joyfully balances her new momma hood with her weekends as a drummer with her girlfriends’ garage band. She simply glows with small town sunshine.

There is a sparkling, revelatory moment, when Mavis, watching the band on a Friday night gig, sees the joy the wife has at banging the drums with her mates, and how much Buddy, her husband, just adores her. The look on Mavis’ face tells it all.

Then we anxiously wait for the moment we know is coming, and it comes like a mid-western summer storm. Mavis, in a desperate last-inning move, and stoked with three glasses of bourbon, puts on her best success costume and comes to the couples’ baby-naming party.

Here, she corners Buddy in a back room and lays out her new life for them.

This leads to an earthquake scene in the driveway in front of everyone. It’s one of those moments people talk about years later. This, and Theron’s final drunken scene in Matt’s house, are of the best, most powerful scenes we’ve seen in recent years. Not since her smashing Oscar turn in “Monster” has Theron so stunned us all.

From “Young Adult,” the film team has brightened each of their paths to glory. Oswalt, who up to now, has been known only as a respected stand up comedian, has just bought himself a brand new career. I love when that happens.

Wilson, as the boyfriend/husband, plays boring like only a professional can. When the holiday crush is over, “Young Adult” will be the last gift to open.

J.P. Devine is a former stage and screen actor.

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