Aileen Wuornos (CHARLIZE THERON) is a near destitute Florida prostitute whose only friend is Thomas (BRUCE DERN), a Vietnam vet. When she first meets Selby Wall (CHRISTINA RICCI) - a young lesbian living with family friend Donna Tentler (ANNIE CORLEY) and her family after her own father disowned her - she wants nothing to do with her.
Yet, the two see kindred spirits in each other and soon enter into a passionate lesbian affair. That doesn't sit well with Donna, prompting Selby to move out and into a motel room with Aileen. The latter, believing she's finally found love, decides to quit hooking and get a legitimate job. When that doesn't pan out and the two realize they need money to survive, Aileen returns to her old profession.
When one such "job" turns horribly bad, however, Aileen ends up killing the "john." With a newfound taste for blood and the desire to make a better life for herself and Selby, Aileen ends up going on a killing spree that eventually draws the attention of the police and threatens to ruin her happiness and relationship with her new lover.