The story begins at the deathbed of good old Ethel Brand, mistress of a noble estate in Surrey. She extracts a promise from her devoted companion, Margaret Walsingham, to lead her wayward grandson, St. Udo Brand, back from ruin through marriage, bequeathing the estate to Margaret if he refuses. When the proud and cynical heir arrives, he insults Margaret and rejects the union.
Soon afterward, he leaves the country for the American war, where he is eventually reported dead in battle. Margaret refuses the inheritance, preferring to earn her living independently, and finds herself under the protection of the Marquis of Ducie after saving his daughter, Lady Juliana, from a railway disaster. Meanwhile, a dark-faced stranger named Roland Mortlake slips into St.
Udo's place, committing murder to assume his identity and claim the Brand estate through marriage to Margaret. Margaret gradually uncovers the chilling imposture, realizing the man courting her is a dangerous imposter and the assassin of the true St. Udo. Despite her terror and lack of allies, she begins gathering proofs of his villainy.
When Mortlake discovers she has found his secret memorandum book, he stalks her relentlessly, attempting to silence her permanently. The tension culminates in a harrowing night attack at Castle Brand, where Margaret narrowly escapes death. Ultimately, her unwavering courage and determination prevail, setting the stage to unmask the impostor and bring him to justice.
