Knowing that star Charlize Theron is one of the producers of this hard-edged, semi-fictionalized story taken from the life of serial killer Aileen Wuornos, one might want to call it a vanity project. But given how the famously beautiful Theron has undergone a Robert De Niro-as-Jake-LaMotta type physical transformation into the pudgy, stringy haired and terminally grimacing Aileen, vanity is 180 degrees in the opposite direction from the goings-on here.

If you've seen the two Nick Broomfield documentaries on Wuornos (especially Aileen: Life and Death of a Serial Killer from last year, a film that serves as a non-fiction template for this one), you will be familiar with the narrative. Horrifically abused as a kid, Wuornos began hooking in her early teens, making her way from her Wisconsin home down to Florida, where she serviced truck drivers and lived a near-homeless life.

After she kills her first john - shown here as an act of self-defense, which it probably was, but, regardless, is in keeping with the film's overall point of view of Aileen as victim more than victimizer - and starts a relationship with lesbian Selby (Christina Ricci), the killings start coming with greater frequency. In an attempt to provide a life for herself and Selby, Wuornos killed seven men over a nine-month period in 1989-90, was caught and eventually executed in 2002.

Theron has probably suffered throughout her career from the treatment many intelligent and beautiful woman receive at the hands of a male-dominated and still sexist world - not being taken seriously for the things that matter more than looks. Monster is an obvious and successful attempt to give the finger to those who haven't taken Theron seriously as an actress. To put it simply, she is unrecognizable, haunting and completely credible as a disturbed, perpetually unloved woman for whom even a genuine smile comes across as an expression of pain.

If Theron is good, Ricci, as the shy young woman from a strict background coming to terms with her sexuality, is even better. Her performance as Selby is the best piece of acting I've yet seen from the crop of adventurous young women - Scarlett Johansson and Julia Stiles foremost among them - set to shake up the Hollywood status quo.

What keeps Monster from being very good, instead of just a movie to see for the acting, is the filmmakers' decision to wholeheartedly make Aileen a victim and the movie an overt - and, unfortunately, reductive - feminist tract, one that occasionally eschews psychological complexity in favour of sermonizing. Anyone who saw Broomfield's last film can't help but see that Wuornos was seriously disturbed. In Monster - the title is meant to be ironic - she is guilty of little more than trying to find and keep love.