Nina has been chosen to play the Swan Queen in Swan Lake, but Leroy, the director, wants her to improve her Black Swan. For those who don't know, Swan Lake is about a beautiful princess, Odile, who is turned to a white swan by a sorcerer. A prince comes and falls in love with her. He is supposed to help her change into human again. However, the sorcerer sends Odette (the black swan) to seduce the prince so that he forgets Odile. In her desperation, Odile throws herself from a cliff and dies.
Leroy says that Nina is a perfect Odile because she is beautiful, virginal and pure; but as the white and black swans are played by the same person, Nina must learn about seduction so she can be a perfect black swan.
Throughout the movie, Nina's distress is obvious. She is afraid Leroy will give the role to her rival Lily if Nina cannot play the black swan well. She has hallucinations, which are getting worse and worse. As Leroy says: "The only obstacle between you and the role is yourself." Her transformation as the black swan is wonderfully depicted in the movie. In the end, Nina plays the perfect Swan Queen, but she also finds her demise.