St. Petersburg, 1881. A winter's night in the palace of Count Vladimir Andrejevich
Princess Fedora, who is to marry the Count the following day, arrives and sings of her love for him, unaware that the dissolute Count has betrayed her with another woman. The sound of sleigh-bells is heard, and the Count is brought in mortally wounded. Doctors and a priest are summoned, and the servants are questioned. It is proposed that Count Loris Ipanov, a suspected Nihilist sympathizer, was probably the assassin. De Siriex (a diplomat), and Grech (a police inspector) plan an investigation. Fedora swears on the jeweled Byzantine cross she is wearing that Count Andrejevich's death will be avenged.
Fedora has followed Loris Ipanov there to avenge her fiancé's death. There is a reception at Fedora's house. A virtuoso pianist is playing for the party-goers. Ipanov arrives and declares his love for Fedora. She tells him that she is returning to Russia the following day. Loris is desperate because he has been exiled from Russia and cannot follow her. He confesses to Fedora that it was he who had killed Count Vladimir. Fedora asks him to return after the reception is over to tell her the whole story. When she is alone, Fedora writes a letter to the chief of the Imperial Police in Russia accusing Ipanov of Count Vladimir's murder. Loris returns and confesses that he killed Count Vladimir because he had discovered that he and his wife were lovers. The night of the homicide, Ipanov had discovered them together. Vladimir shot at Ipanov and wounded him. Ipanov returned fire, killing Vladimir. Fedora realizes that she has fallen in love with Ipanov, and that he killed not for political ends, but to defend his honor. They embrace and she convinces him to spend the night with her.
The Bernese Oberland in Switzerland
Loris and Fedora are now lovers and living in her villa. With them is her friend, Countess Olga Sukarev. De Siriex arrives to invite Olga on a bicycle ride. He tells Fedora that as a result of the letter she had written to the police chief, Loris's brother, Valeriano, was arrested for being part of the plot to murder Count Andrejevich and imprisoned in a fortress on the Neva river. One night the river flooded and the young man was drowned. When Loris's mother heard the news, she collapsed and died. Fedora is anguished - she has been the cause of two deaths. Loris receives a letter from a friend in Russia who tells him of the deaths of his mother and brother and that the cause was a woman living in Paris who had written a letter denouncing him to the police. Fedora confesses to Loris that she had written the letter and begs his forgiveness. When he initially refuses and curses her, Fedora swallows poison which she had hidden in the Byzantine cross she always wore around her neck. Loris begs the doctor to save her, but it is too late. Fedora dies in Loris's arms.