Secretary

Lee Holloway is a young, troubled woman who has just left a mental hospital. She returns with her family, but we can easily see that it isn't the best environment to keep her sane. She notices it as well and tries to achieve an escape dating a comfortable next-door boyfriend and taking a job as a secretary for Edward Grey, a local lawler.
The professional relationship between the perfectionist Grey and the eager to please Holloway grows into something deep and strange as Grey's repressed, sadistic sexual tastes take over him.

I recommend this film, first, for it is a rather good one with a great acting. And, second, because the rather realistic way a SM relationship is portrayed.