Alternatively darkly funny and acrid, Young Adult functions most strongly as a vehicle for a performance by a beautiful movie star who’s not afraid to be ugly on every level. Of course, Charlize Theron going ugly physically is what most women would hope to look like in their dreams. That said, Theron’s Mavis (seriously—Mavis?) is ugly inside, too—selfish, perhaps mentally ill, and pathetic. Happily, Theron keeps her that way without finding a way to let the audience know that she’s just acting. It’s a daring and brave performance, but it isn’t enough to hold the film together.

Diablo Cody’s screenplay is like Juno in its snark and snappy, intelligent one-liners, but unlike Juno and Juno, doesn’t go back to a new normal at the end, leaving things a bit up in the air. The set-up is rife with possibilities—big-city girl with some success returns to her hometown to reclaim old flame, now married and a new dad—and fulfills some of them. But other than Mavis’s budding new relationship with town gimp Matt (Patton Oswalt), the nothing solid forms along the way. It’s set up for Mavis to get her comeuppance, but she never does. Matt is all set up to learn some harsh but necessary lessons from Mavis, but apparently he never does either. Happily, Matt is the clear moral center of the film for a while, and his observations and back-and-forth with Mavis are the best moments in the film. While we either revel in Matt’s comments or relate to them, we merely watch Mavis’s and respond by being appalled or distanced from her character.

In Juno, we knew who was the smartest person in the room—Juno. She could be wicked with her tongue, but she was a high-schooler (we understood she was young and excused a lot of the talk) and she generally meant well. She was surrounded by real characters except for the comic turns of the abortion protester and the pregnancy clinic secretary. The dad, the stepmom, the boyfriend, even both prospective adoptive parents—these kept the film grounded. Matt’s character can’t bear that weight alone here, and the film tends to spin off its axis as a result. The old high school friend who was crippled in a car wreck, for example, is just another chance to mock with no seeming purpose. It might have provided at least a suggestion of a teaching moment for Matt had the friend been more than a personification of “positivity”; Matt certainly needed to learn how to move on, and that might have lent a certain depth to the film. Instead, we just roll our eyes again at another jerk.

Buddy Slade (Patrick Wilson) has a great name for an old jock flame. But he is both underwritten by Cody and underperformed by Wilson. Slade is more of an idea or plot contrivance than a real person. His actions are immature or senseless for a supposedly happily married man (meeting an old flame for drinks?), and he acts especially clueless for someone who is not supposed to be. Wilson is a capable enough actor, but his character, so highly anticipated in the script, turns out to be soft and edgeless. It’s a huge missed opportunity.

Then there is that last conversation between Mavis and Matt’s sister Sandra (Collette Wolfe), which takes the cool city/dumb small town tension so structural to the plot and twists it beyond recognition, leaving us confused as to what we’re supposed to be taking away from the film. Are Cody and director Jason Reitman reluctant to go all the way with their criticisms of either the big city or the small town, Mavis or the folks who stayed and settled where they went to high school? No one comes out unscathed, but no one person or idea comes out a winner either. No one has to, but please at least suggest some possibilities of a closing perspective. The film has taken pity on Mavis, but clearly realized the folly of her actions with Buddy. The townspeople, on the other hand, are sincere and just a little stupid, (apparently) but gain some respect as Mavis deconstructs. And yet we have an ending that appears to turn all that on its head. The big city is for the cool and beautiful people, no matter how nuts, and the level-headed small town folks should be wise enough to stay put and know their place.

The film starts brave, occasionally goes off the road, and appears to lose its caustic nerve at the end. It starts out snarky; it should have ended the same way.