Monster (18)

Based on the real-life story of the prostitute and murderer Aileen Wuornos, Monster is a disquieting portrait of a soul in stress. The movies are overrun with serial killers - Hollywood seems to believe there is no other kind - but after her 1992 trial, Wuornos made herself notorious as the first ever woman to earn the title. Nick Broomfield made two documentaries about her, the second of which - Aileen: Life and Death of a Serial Killer - he filmed just before she was executed in October 2002. It argued convincingly that she was mentally disturbed, and thus not fit to stand trial.

Monster, written and directed by the first-timer Patty Jenkins, presents Wuornos's life as a murder mystery, not in the way that casts doubt on her crimes (she shot dead seven men), but in the sense that murder is always a mystery. The film is set in the year or so before her arrest in 1990, and aims to supply a psychological background to the feral and somewhat frightening woman Broomfield got to know on Death Row.

If the call had gone out to find an actress for the role of Wuornos, it is hard to imagine Charlize Theron making anybody's short list. A former dancer and model, the South African-born Theron once featured in a memorable minor role in Woody Allen's Celebrity, memorable principally because of her blonde-goddess untouchability. Otherwise, she has been seen slumming through dismal stuff like The Legend of Bagger Vance and Sweet November.

Well, bravo to the casting director who dared to suggest her for this, because as Wuornos she is sensational to the point of eeriness. One tends to go "tsk" at news that an actress has put on 30 pounds and endured hours each day in the make-up chair - oh, the sacrifice! - and when that actress picks up an Academy Award for her trouble, the temptation is to groan. Go on, I did.

The wonder is that Theron achieves not just a physical transformation but an intensely detailed and characterful performance. The wind-burnt, freckled skin, the wildcat stare, the belligerent swagger, every twitch and grimace are pinpoint accurate, yet there is something else in there, something only imagination could have furnished. Remember that we have only ever seen Aileen in prison (or courtroom): Theron gives a more than plausible incarnation of the woman she might have been on the outside. Drinking in scuzzy biker bars and selling herself on the roadsides of Florida, she still looks caged and projects a heart-clutching mixture of aggression and vulnerability.

Jenkins's script is more sketchy than Broomfield about Aileen's childhood - she was sexually abused from the age of eight, and was thrown out by her family after she went "hooking" aged 13 - but we see enough of her adult life to understand why she would respond so passionately to a genuine offer of love. It comes from a young woman, Selby (Christina Ricci), who has been sent to Florida by her fundamentalist father as punishment for her homosexuality.

With Selby peculiarly helpless about work, Aileen has to keep them both, which means continuing to trawl for johns - male clients - on the dark and violent highways. One night, she picks up a brute who beats her almost to death; struggling free, she empties a handgun into him. It's her turning point. Whether driven by a lifetime's disgust or a liberating blood-lust, she takes to killing like a vigilante. And for a while she gets away with it.

The families of her victims will argue otherwise, but it is the film's contention that Wuornos, for all her wantonness, was no "monster". She was a naive, uneducated woman who had suffered horrific abuse all her life and could see no way to escape from its wretchedness. Even Selby, the one person who might have saved her, turned out to be as needy and unreliable as she was (Ricci plays her as a spoilt child).

It's a tribute to Theron's expressive mobility that one feels not abhorrence towards Wuornos, but a terrible sympathy. There is a tragicomic edge to the scene in which she turns up for a job interview at a law firm without references, resumé or indeed the smallest notion of what "a job" might actually entail. And the moment her interviewer turns his nose up she unleashes a ferocious volley of expletives. Back to the highway for her.

What, in the end, do we make of Wuornos? "I feel like I never had a choice," Aileen says, and there's no arguing that her family and fate contrived to make her an outlaw. But a killer too? Patty Jenkins offers two scenes of heartfelt anguish that thrum with ambiguity. Wuornos takes a drive with a john who turns out to be a shy stutterer with no taste for violence. Later she picks up a man she mistakes for a john - he's actually an old married geezer who offers to help her. One of them she spares, and one she murders. It's because she knows she has a choice that she feels so tormented.

Monster doesn't make it easy to judge her, though it's been a long time since a multiple murderer prompted such a terrible surge of pity. "I always wanted to be in the movies," she says in voiceover at the start. She got her wish, but they were movies she couldn't have envisaged in her worst nightmare.