Sherlock Holmes is a brilliant private consulting detective operating out of London. He applies keen powers of observation and deductive reasoning to solve complex mysteries. For instance, he deduces his new client's mode of transportation simply from the mud stains on her sleeve. He takes on cases for the intellectual challenge rather than monetary gain, approaching his investigations with a methodical, medically trained precision.

Dr. Watson is a medical doctor and the devoted friend of Sherlock Holmes. He shares the Baker Street flat and chronicles the detective's remarkable cases. He provides a grounded, accessible perspective to the complex investigations. He willingly faces unknown dangers in the dead of night to assist his brilliant colleague.

Dr. Roylott is a towering, ill-tempered medical practitioner and the last surviving member of a ruined aristocratic family. Following a prison sentence for beating his butler in Calcutta, he secludes himself at the crumbling ancestral manor of Stoke Moran. He maintains a fierce grip on his late wife's fortune and aggressively terrorizes his neighbors. He controls his stepdaughters' inheritance, which they can only claim upon marriage.

Helen is a woman in her early thirties whose hair has turned prematurely gray from intense stress and grief. She lives in terror at the derelict Stoke Moran manor with her erratic stepfather. Haunted by the unsolved death of her twin sister two years prior, she travels to London to hire a private consulting detective. She seeks professional assistance because disturbing noises have resumed at the manor following her recent engagement.

Julia is Helen's twin sister who lived under the oppressive shadow of their stepfather. She suffered from the same intense stress that aged her sister prematurely. She died from an unexplained cause just weeks before she was supposed to be married.

Mrs. Stoner is the late mother of Helen and Julia. She married Dr. Roylott in India and entrusted him with her considerable fortune. She stipulated that her daughters receive portions of the inheritance upon their marriages. Her tragic death in a railway accident left her girls entirely dependent on their unstable new stepfather.

Percy is a long-time friend of Helen Stoner who recently proposed to her. He dismisses her mounting dread entirely. He refuses to acknowledge the reality of the threats she faces at the Roylott estate.