Charlize Theron came on the scene in 1996, with small roles in “That Thing You Do!” and “2 Days in the Valley.” She has appeared in 17 more films since then. I have reviewed 11 of them. I can recall her performance specifically in almost none of them. I said in my review of “Sweet November” that she employs a “featherweight acting technique (that) makes her characters all seem believable but shallow.” That sounds about right.

Her performance in “Monster” would be brilliant anyway, but it’s all the more surprising given her track record as a reliable but unimpressive actress. In “Monster,” she doesn’t just portray Aileen Wuornos, the serial killer/prostitute who murdered seven men in Florida in the ’80s; she becomes her. I believe every second of her performance, every jitter, every burst of white-trash belligerence, every desperate act of loneliness and fear. It’s one of the most convincing and riveting performances I’ve ever seen.

The story in “Monster,” written and directed by first-timer Patty Jenkins, is true, and it’s the sort of lurid stuff that could turn bad easily. Aileen, a part-time whore in the Daytona area, is on the verge of suicide when she stumbles into a bar and meets Selby (Christina Ricci), a girl whose father sent her here from Ohio to save her from a life of lesbianism. (It didn’t work; it’s a gay bar she’s hanging out at.) Aileen is straight, but she doesn’t have much use for men anymore other than financially, so she lets the smitten Selby buy her drinks, and they become friends.

A romantic relationship develops, including an erotically charged encounter at a skating rink that hints at Jenkins’ directorial ability, and soon the two are making plans to strike out on their own together. Aileen continues to prostitute herself to support them, and it is during a sexual transaction gone awry that she winds up killing a client and making off with his car and money.

Thus begins the nightmarish downward spiral that comprises most of the remainder of the film, and I marvel at Jenkins’ restraint in attempting to neither demonize nor excuse Aileen.

Christina Ricci, though not as flashy as Theron, personifies the skillful supporting performance, adding strength to the picture’s foundation. Bruce Dern is also notable as a bartender friend of Aileen’s.

As I said, though, this is the sort of material that can go bad easily, and Theron’s performance single-handedly saves it from becoming a Lifetime TV movie. Her Aileen is so real and vivid that the film remains compelling even when Aileen’s actions mystify or horrify us. Charlize Theron was already a prolific actress, but with “Monster,” a star is born.

A- (1 hr., 51 min.; R, abundant harsh profanity, some graphic violence, some very strong sexuality, some nudity.)