In London's backstreets, twin sisters Isadora and Marisa live a modest life with their mother. When their mother dies in an accident, Isadora searches through her belongings and discovers evidence that Marisa is the missing daughter of the Earl of Strathford. Envious and ruthless, Isadora alters the evidence and impersonates the earl's daughter, successfully entering high society as a debutante. She revels in the attentions of wealthy young men and a crown prince, while Marisa, allowed to live at Strathford Castle by the earl's goodwill, is reduced to a servant and subjected to Isadora's cruelty.

The earl and his butler Crocky gradually sense something wrong with Isadora's character and are drawn to Marisa's innate grace and kindness. Meanwhile, Marisa's presumed-dead mother Francoise is found alive but suffering from amnesia and is brought to the castle, deepening Isadora's anxiety. To eliminate Marisa, Isadora hires the hypnotist Tutankhamen, who places Marisa under a spell that convinces her she does not belong at the castle. Marisa leaves to work as a nurse in war-torn Biafra, Africa. There the hypnosis breaks, and she marries Dr. Cliff Montgomery, giving birth to a daughter, Maria. Isadora, after a string of affairs, marries a supposed millionaire who is actually Tutankhamen in disguise. His true goal is to use a secret passage from Strathford Castle to Buckingham Palace to harm the British royal family. When Isadora learns this, she kills Tutankhamen and publicly admits that Marisa is the true earl's daughter.

Marisa returns to the castle and raises Isadora's daughter, Muse, as her own. Years later, Isadora returns, her beauty destroyed by illness and her mind unstable. She tells Muse that Marisa stole her place, and the two vanish. The earl's family faces bankruptcy, and Marisa and Maria travel to the family's ancestral estate on Glasgow Island to resolve the crisis. The islanders bear a generations-old grudge against the earls, and Isadora and Muse have taken control of them. A witch trial nearly leads to Marisa and Maria's execution, but Muse, who loves Maria, saves them. Isadora, overcome with remorse, sets fire to the manor and dies in the flames. From the ruins emerges a diamond castle, which Marisa gives to the islanders, achieving reconciliation. Muse enters a convent, choosing to pray for her mother's sins and for Maria.