A young woman finds herself ensnared by a malevolent occultist who seeks to use her life force for his dark experiments. Arthur Burdon, a surgeon, attempts to protect his fiancée, Margaret Dauncey, from the sinister influence of Oliver Haddo, a man who claims to possess supernatural powers. Haddo utilizes psychological manipulation and arcane rituals to isolate Margaret from her social circle and her fiancé. The narrative unfolds through a third-person perspective that shifts between the clinical observations of the medical professionals and the increasingly bizarre occurrences surrounding Haddo's activities. The story explores the tension between rational science and the irrational claims of the occult.

Readers frequently highlight the contrast between the clinical, detached prose of the medical characters and the flamboyant, unsettling nature of the antagonist. Discussion often centers on the author's ability to maintain a sense of dread without relying on overt supernatural elements. Critics note that the inclusion of the autobiographical fragment offers a unique look into the inspiration behind the character of Oliver Haddo. The work is often analyzed for its commentary on the dangers of unchecked ego and the vulnerability of the human psyche to manipulation. Many readers appreciate the atmospheric tension that persists throughout the narrative.