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The young German princess Sophia Frederica is chosen by Queen Elizabeth of Russia to marry her nephew, the "royal halfwit" Peter, in order to provide a sane male heir to the throne.  Elizabeth changes Sophia's name to Catherine and, following the wedding, Peter tells the Queen he hates his wife.  When the marriage remains unconsummated, Catherine is blamed.

Meanwhile, the debonair Count Alexei pursues Catherine and gives her a locket that contains his picture.  The Queen discovers their rendezvous, however, and demands that Catherine not leave her sight.  That night, Catherine discovers Alexei is the Queen's lover and throws his locket out her window, but in remorse goes out into the snowy night to find it.  There she meets Lieutenant Dmitri, a guard, who does not recognize her.  In an act of vengeance, Catherine makes love with him and becomes pregnant.

When she gives birth to a boy, Catherine satisfies the Queen, but Peter knows the child is not his and his hatred for Catherine grows.  When the Queen dies, Peter becomes czar and begins a reign of terror, torturing and murdering thousands of Russians.  By befriending much of the royal army through her feminine wiles, Catherine builds her defense against the increasingly insane Peter.  When Peter has Catherine arrested in order to bring his mistress to the throne, Catherine escapes and, with the help of General Orloff, storms the palace, after which Orloff murders Peter.