Arthur Burdon, a London surgeon, arrives in Paris to visit his fiancée, Margaret Dauncey, a beautiful young woman studying art. Arthur has been Margaret's guardian since the death of her father, a family friend who left her penniless, and he secretly supported her for years before proposing. He insisted she spend two years in Paris before marrying. Walking through the Luxembourg Gardens with Dr Porhoët, an elderly French physician who spent most of his career in Egypt, Arthur encounters Oliver Haddo, a large, showy Englishman whom Dr Porhoët met at the Arsenal library while researching the occult. Haddo claims to be a magician.
Margaret shares a flat near the Boulevard du Montparnasse with Susie Boyd, a plain, witty, 30-year-old former schoolmistress who serves as a maternal companion. When Arthur visits, Susie is struck by his capacity for suffering and warns Margaret to be careful with him.
The group dines at the Chien Noir, a bohemian restaurant in Montparnasse, where Haddo arrives uninvited. Enormously fat, with pale blue eyes that give the unsettling impression of looking through people, he insults the regulars with elaborate wit and drives most away. Margaret reacts with instinctive physical revulsion. At a nearby fair, Haddo leads the group into a snake charmer's booth, where he lets a horned viper bite his hand and heals the wound by muttering over it. He proves the venom is real by having the snake strike a rabbit, which dies instantly. Margaret cries out at the cruelty and flees.
Arthur writes to his friend Frank Hurrell in London for information about Haddo. Meanwhile, the group visits Dr Porhoët's apartment, where the doctor displays his occult library and recounts an experience from Alexandria: A sheikh used a magic mirror of ink on a boy's palm to produce visions of Dr Porhoët's mother dying in Brittany, later confirmed as true. Hurrell's reply confirms Haddo's distinguished family and Oxford education but details his cruelty, dishonesty, and a camp servant's suspicious death during an expedition. Hurrell warns Arthur to avoid him entirely.
At tea in the studio, Haddo speaks with sudden passion about the alchemists' true goal: not gold but absolute power, including the creation of life. He describes homunculi, artificial living beings supposedly generated in the 18th century. Margaret's terrier attacks Haddo, biting his hand; Haddo kicks the dog savagely. Enraged, Arthur punches and kicks him. Haddo offers no resistance, delivers an abject apology, and leaves. Susie catches a fleeting expression of satanic hatred on his face, followed by a smile she finds even more terrifying.
Haddo then engineers a scheme to isolate Margaret, sending a fake telegram that lures Susie to the train station. While Susie is away, Haddo feigns a collapse in the courtyard, and Margaret helps him into the studio out of compassion. Once alone, he recovers and speaks seductively about art, plays the piano with extraordinary skill, and burns substances whose vapors plunge Margaret into a visionary trance. She sees vast landscapes, historical figures, and scenes of terrible lust, and she emerges weeping and ashamed. She lies to Susie about the visit.
Despite her revulsion, Margaret finds herself irresistibly compelled to visit Haddo at his apartment. He conjures visions of Eastern freedom and supernatural knowledge that make her despise conventional marriage. She surrenders and returns daily, experiencing ecstasy mingled with loathing while lying to Arthur and Susie. When Haddo announces he is leaving Paris, Margaret agrees to marry him after a desperate struggle. On Arthur's birthday, she dines with him in radiant beauty, kissing him with unprecedented passion while consciously savoring the cruelty. The next morning she marries Haddo at the British Consulate and departs for England, sending Susie a brief note.
Arthur is devastated. Susie confirms Margaret's premeditated deception: The trousseau was sent ahead, and bills were paid with Arthur's money. Dr Porhoët arrives with a mocking telegram from Haddo. They suspect Haddo orchestrated Margaret's marriage as revenge for the beating. Arthur departs for London, asking Susie to tell Margaret he bears no ill-will.
Months pass. In Rome and Monte Carlo, the Haddos live extravagantly amid rumors of orgies, satanic ceremonies, and attempts to create living creatures. The marriage is rumored to remain unconsummated, with Haddo using Margaret's virginity as a talisman. Susie visits London and finds Arthur terribly aged. At a chance supper party, the two groups meet. Margaret, drinking heavily, tells a grossly indecent story that shocks the table.
Arthur later visits Margaret privately. She confesses that Haddo never loved her and gained power over her the day she helped him in the studio, when his collapse was feigned. Her will has been destroyed. She believes Haddo intends to use her for a magical experiment requiring her life. Arthur takes her to Susie, who brings her to a cottage in Hampshire. A divorce suit is filed, but as the trial approaches, Margaret grows agitated. One morning Susie finds her gone, with a note saying she could not help herself and has returned to Haddo.
Weeks later, Arthur visits Skene, Haddo's estate in Staffordshire, and finds Margaret at a stone bench in the neglected park, shockingly aged. She tells him Haddo wants her life for his experiment and urges him to marry Susie. Arthur returns to Paris, then is seized with a conviction that Margaret is in mortal danger. He insists they all go to England. At an inn in the village of Venning, they learn Margaret is dead, buried that morning of heart disease. Arthur suspects murder but cannot convince the local doctor to support exhumation. He confronts Haddo at Skene, but Haddo deflects with mocking calm.
Arthur reveals to Dr Porhoët that he now remembers being the boy from the Alexandria experiment years ago and asks the doctor to perform a necromantic invocation at the stone bench. After fasting, they go at night. Porhoët traces figures, burns incense, and recites invocations while a violent supernatural wind rages around them. Porhoët calls Margaret's name three times. The wind ceases, and they hear the unmistakable sound of Margaret weeping. Arthur sees her on the bench, confirming that she did not die naturally.
Days later, during a storm at the inn, the lamp is extinguished and Susie senses an intruder. Arthur grapples with the figure in darkness, strangles him, and breaks his arm. When the lamp is relit, no body is found. They go to Skene immediately. Forcing entry, they ascend to hidden attic laboratories blazing with lamplight and furnace heat. In huge glass vessels they find living abominations: pulsating masses of flesh, mummy-like forms, a creature with two monstrous heads, and a four-foot being with an enormous skull that gibbers and beats its human hands against the glass. These are homunculi, the results of Haddo's attempt to create life.
Behind the tables, Haddo lies dead, his face purple, finger marks on his throat, and his right arm broken in the same place Arthur broke it during the struggle at the inn. Arthur sets fire to the house. From a hilltop, the three watch Skene burn. Arthur, singularly relieved, puts his arm around Susie's shoulders. She feels ready to forget the terrible past and embrace the happiness that seems at last in store for her. Arthur turns their gaze eastward, where the sun appears upon the face of the earth.