Young Adult (15)

Starring: Charlize Theron, Patrick Wilson, Patton Oswalt

There's excellent acid in the combination of director Jason Reitman and writer Diablo Cody, already proven in the snarky Juno and reinforced by this smalltown comedy of self-delusion.

Young Adult is a very good film. Too bad that it could have been a great one. Charlize Theron does her best work since Monster as Mavis Gary, a 37-year-old divorcee and struggling ghost-writer of teen fiction. On learning that her high-school ex Buddy (Patrick Wilson) has just become a dad, Mavis hits on a determined but deluded scheme: she will return to their home town of Mercury and win him back.

There's a drawback in that Buddy is happily married to Beth (Elizabeth Reaser) and is absorbed in their baby, but Mavis just jumps right in, vamping and flirting like it was the old days ("psychotic prom queen bitch" is how other contemporaries remember her). Cody's characterisation of her sails within an ace of outright masochism, yet it's pulled back into comedy by the presence of Matt (Patton Oswalt), once the fat kid at school who got beaten so badly he now walks with a stick. He's also the guy who never left hicksville, the sort that Mavis despises, yet a shared love of bourbon and his handiness as a confidant unite them.

In tracing this arc of desperation, neediness and envy, the film keeps edging towards the dark – she has a hair-pulling problem, an image problem, a drink problem – which makes you wonder what sort of ending it's heading for. It surprises by not having an ending, just pulling up bafflingly short by at least two or three big scenes. I kept wishing it hadn't, because Cody's writing is super-sharp and the comic sparring between Theron and Oswalt is engagingly nasty. But there you have it, a brilliant first hour, and then a hopeless fizzle.