Minor spoilers behind the cut.
It's rare that a movie lives up to its hype, but this one did. Black Swan is incredible enough that I have few words for how good it is, or for how effective and moving the performances are.
The story of Swan Lake, arguably the most famous ballet of all time, derives from Russian fairy tales, and like all enduring fairy tales, the story moves people in a way that has inspired them to write their own versions of the story over the years, one of which is this new take. It's a Jekyll and Hyde story (with other themes, but it's the "dual nature" aspect that Black Swan uses) made before the original Jekyll and Hyde. In it, a pure woman named Odette is cursed by an evil sorcerer named Rothbart; she becomes a white swan, and only true love can break her spell. Unfortunately, though a prince named Siegfried falls in love with her, Rothbart's daughter, the black swan Odile--always danced by the same ballerina as Odette--seduces him away before he can save her. Devastated, Odette kills herself, and in death finds the freedom Siegfried didn't/couldn't give her in life.
Black Swan is the story of Nina Sayers, a New York Company ballerina who lives for ballet; she doesn't date or go out, lives with her mother, and her room is that of a little girl with pink hearts and stuffed animals. But when she's promoted to play Odette/Odile in a new Swan Lake production, just how devoted to her art she is becomes horrifyingly evident, as does her naiveté about the world outside the studio.
Sex is a notable theme in the story, and as duality is THE theme as far as I can see, Nina is pulled two ways about it. On one side is her mother, determined to keep her a perfect little girl forever, punishing her when she goes out, keeping friends from coming inside, using guilt to keep Nina pure and docile. On the other hand there's Nina's scuzzy ballet director, used to the privilege of sleeping with any dancer he deigns to gift with a big role. When Nina is uncomfortable, he gets angry that she's not turning him on and makes fun of her, telling everyone she's frigid.
And how can Nina fight them? Her mother is Nina's whole world, and the director is a godlike figure, one who turns her on. A well-adjusted woman would see him for the scum he is, but because Nina is so inexperienced, she's drawn to his attention and sexuality. Nina's release is dance, and her resolve to perfect her performance--and, really, herself, though she doesn't seem to see that--leads to increasingly gruesome scenes of injury and self-harm committed in the name of art. By the time otherworldly things start to happen we in the audience are so caught up in Nina's mad focus on beauty it's impossible to say whether supernatural happenings are driving Nina insane...or if Nina's insanity has so drowned her that she's making strange things happen.
Black Swan is a visual movie, one best seen on a big screen. The dialogue is great but secondary. For example, in one scene the director gives a toast to beauty, then we see one dancer almost kill herself with hysterics when she's treated cruelly and Nina's skin tear as she trains past her body's endurance point in a visual display of irony. It is a horror story, if not just a horror story, and will scare the crap out of you in parts, stun you in others, and should disturb you. Most importantly, whether you like it or not, I can almost guarantee you won't forget it.
I could not recommend this more highly. I almost never see movies full price, but the big screen is really the only way to see this one. You will do yourself a disservice not seeing it.
If you like this, another good example of a Swan Lake inspired story is Matthew Bourne's new ballet Swan Lake, created in 1995, which is the story of a young prince who falls in love with a white (male) swan...but then the kind white swan becomes a cruel black swan that drives him mad. It hasn't been off stage since it's introduction, it's so popular and critically lauded. The themes are very similar to this new movie.