St. Petersburg, Russia, 1745. Grand Duke Peter, heir to the throne, is furious at the plans of his aunt, the reigning Empress Elizabeth, to marry him to the young German princess Sophia-Augusta von Anhalt-Zerbst. He confides his low opinion of the Empress to his friend Gregory Orlov, who reminds him that he must control his temper and remember that he will one day become Emperor of Russia.

In the Palace, the Empress is angry with her attendant and lover Bestujhev, who questions the wisdom of the marriage. She throws crockery at him and he leaves, returning to offer his resignation, which she refuses. Grand Duke Peter arrives and tells her he will not marry. They quarrel and he storms out, at which point the Empress faints.

Meanwhile, Princess Sophia and her mother are waiting to meet the Empress. Hearing about the Grand-Duke's refusal, they start to leave, when Sophia is separated from her mother. Lost, she runs into Peter, who hides his identity and talks with her. Not realising who he is, she tells him she is not interested in the throne, but has loved the Grand-Duke since she was a child. Charmed, he leads her to the Empress, where he announces he will marry her after all.

On the day of the wedding, Peter becomes convinced that Sophia, now renamed Catherine, has tricked him. After the ceremony, he leaves her alone to visit his mistress. The Empress is disturbed with her new lover by a maid, who tells her Catherine is crying alone on her wedding night. Furious, Elizabeth goes to Catherine to comfort her.

Two years later, Catherine is inspecting her regiment, with whom she flirts outrageously. Peter is furious, and drags her away. He confronts her with her unfaithfulness, and she confesses to having seventeen lovers. He drags her to the Empress, who takes Catherine to one side. Catherine admits she made up her lovers to make Peter jealous.

The Empress consents to a divorce, but Peter's interest is growing, and he asks her to dine with him. She admits her deception, and the divorce is called off.

Some time later, a ball is in progress. The Empress is ill. Catherine impresses her with her understanding of matters of state. Meanwhile, Peter is already making preparations for his aunt's death. Defiantly, Elizabeth makes a show of good health, but the strain is too much for her and she faints.

On her deathbed, Elizabeth bemoans Peter's madness and expresses her regret that he, not Catherine, will rule Russia. Meanwhile, Peter is already preparing his first acts as Emperor: new coins, new uniforms for the military, the return of all those exiled to Siberia, to be replaced by those who were in favour with the Empress.

After the Empress' death, Peter's behaviour becomes increasingly erratic, and begins to draw the concern of his ministers and the military, and they beg Catherine to take his place.

Peter has Catherine moved from her quarters to another place in the palace, and installs his mistress in her place. At a state function, Peter humiliates Catherine in front of visiting dignitaries. Later, Orlov confesses his love for Catherine. Reluctantly, she agrees to overthrow Peter, provided that he is not harmed.

While Catherine delivers her victory speech to the delight of the crowds, Peter is captured and exiled. As Catherine celebrates however, Orlov tells her that Peter has been killed. Catherine is devastated, complaining that Peter's death is too high a price to pay for the throne. But Russia, she is told, wanted her at any cost.