Princess Ojeta is turned into a white swan by a demon at Swan Lake. Prince Siegfried swims Swan Lake and falls y in love with Ojeta. On the night when the prince chooses his bride, the Devil has his daughter, the Black Swan, disguise herself as Ojeta to deceive the prince.
The prince is almost deceived, but eventually realizes in time and fights the demon, killing it. The White Swan is restored to her original form as a princess and is united with the prince in a happy ending.
Tchaikovsky's ballet Swan Lake, in 4 acts, was written in 1876. It was the first dance piece made by Tchaikovsky and is based on folklore.
Expanded:
Compositional Background
1871 In 1871, at the age of thirty-one, Tchaikovsky came to stay with his sister. Being the good uncle he was, he decided to give his lovely nephews a special gift.
One-act ballet music based on the children's usual reading of the fairy tale "Swan Lake" by the German composer Moltzeus (which tells the story of how a young knight defeats the devil, rescues a maiden who has been magically transformed into a swan, and finally marries him).
In 1875, the composer's close friend and artistic director of the Bolshoi Theatre, V. Begichev, asked Tchaikovsky to compose music for a large-scale dance libretto, "Swan Lake," promising an honorarium of eight hundred rubles upon completion. For Tchaikovsky, who taught at the Moscow Conservatory on a salary of only 1,500 rubles a year, this was an enormous sum.
But, of course, it was also his desire to do something bold in the field of ballet that motivated him to accept the commission, and that is how we came to hear some of the most popular ballet music ever written.
Baidu Encyclopedia - Ballet Swan Lake