Based on the true story of Aileen Wuornos, who had the dubious honour of being dubbed "America's first female serial killer", the film picks up the story of her life sometime in the mid-late 80's, just before she embarked on her killing spree and looks at the events that drove her from prostitution to murder as a means of survival.
It is evident from the opening monologue, that Wuornos (portrayed here by Charlize Theron, looking very unlike her usual self) has lived a very troubled life. But things appear to take a turn for the better when she develops a relationship with a young lesbian named Selby (as played by Christina Ricci).
Unable to find regular work, Aileen finds herself having to turn back to prostitution in order to support them. But after being roughed up quite badly by one client too many, she discovers murder is a much simpler and quicker method of earning money, as the film follows her downward spiral into the events that lead up to her arrest.
Directed by a then unknown Patty Jenkins, this gritty disturbing look at one of America's infamous serial killer's remains, with one or two minor exceptions, largely faithful to the original case and though not excusing her actions, manages to paint a somewhat sympathetic picture of her background and provides a fascinating insight as to why she became the "Monster" of the films title.
The film also benefits from the performances of Charlize Theron and Christina Ricci, and a solid supporting cast which includes 70's actor Bruce Dern. Definitely one of the better serial killer biopics to have been released.