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In 1872 London, surgeon Sir Joel Cadman visits his former student, Dr. Gordon Ramsay, who is awaiting execution for the murder of a moneylender named Curry. Ramsay insists that he is innocent, but has no memory of that night and does not believe the body pulled from the Thames was Curry. Cadman urges Ramsay to drink an ancient Indian potion called the "black sleep" that will enable him to feign death before he is hanged. Because his body will be entrusted to Cadman, Cadman promises to administer a timely antidote. The next morning, Ramsay is pronounced dead in his cell. Later, Cadman and his gypsy assistant, Odo, administer the antidote in Cadman's office. Cadman then explains to Ramsay that he needs a skilled surgeon to help with important work and will hide Ramsay at his estate on the coast while Odo arranges for Ramsay's mock funeral. Arriving at Cadman's estate, Ramsay is startled by screams from a young woman named Laurie, who is being attacked by a large man. Cadman's nurse, Daphne, calms the man, called Mungo, and takes him away. Later, Cadman and Daphne secretly visit the bedroom of Cadman's comatose wife Angelina, where the doctor vows to find a cure for her. The next day, when Ramsay observes that Mungo resembles one of his former professors, Dr. Munroe, Cadman reveals that Mungo is Monroe, whose mind has been damaged. Cadman states that he wants to help Mungo and others like him, then reveals that he has been performing surgeries to map the functions of the human brain. Showing Ramsay a chart of a brain, Cadman explains that he needs his expert assistance for additional surgeries. Cadman then takes Ramsay to a secret passageway that is accessed through the hearth in his study. The passage leads to a room where an unconscious man is being prepared for surgery by Daphne and Laurie. Ramsay assists Cadman, who demonstrates that touching certain parts of the brain causes motor reactions. When he probes further, however, Ramsay is shocked that cerebral fluid flows from the brain, indicating that the man is not dead, as Ramsay had assumed. Cadman dismisses Ramsay's outrage by saying that the man is in a black sleep and possible brain damage is a risk he must take to advance medical science. That night, Laurie slips a note under Ramsay's door, asking him to meet her. When he does, she says that his reaction to Cadman's operation made her feel she could trust him, and she reveals that Mungo is actually her father, who came to Cadman for an operation to alleviate a partial paralysis but was turned into what he is now. She implores Ramsay to perform another operation to change her father back to his normal self. Ramsay senses that Laurie is telling the truth, but asks her to produce proof of her assertions that there have been other operations. The next day, Ramsay asks Cadman about his operation on Mungo, saying he deduced a surgery from Mungo's head scars. Cadman responds that he was deeply troubled to have caused his old friend harm while trying to cure his paralysis. Just then Odo arrives and talks to Cadman about "Miss Daly," a beggar who had been the mistress of the moneylender Curry. Although Odo's words are oblique, Ramsay also hears the name Curry. That night, in Odo's London tattoo parlor, Miss Daly arrives to answer his request for a model. Odo gives her money to sketch her face, then puts the black sleep potion into her glass of sherry. Later, Scotland Yard Investigative Sergeant Steele rings the bell to inquire about Miss Daly, who was seen going into Odo's shop. Steele says that he is checking on a story she told them about a drunken sailor who claimed to have helped Curry into a cab on the very night that he was supposedly murdered by Ramsay. The sailor has disappeared and they need to confirm her story. Odo feigns ignorance of her whereabouts, and after they leave, spills the antidote rather than administer it. Meanwhile, at Cadman's estate, Ramsay and Laurie discuss Cadman's brain chart. Ramsay has deduced that initials on the chart stand for specific patients and wonders if "C5" could stand for Curry. Laurie then relates an odd joke Cadman made during C5's operation, commenting that "curry powder" and the black sleep both come from India. They then decide to go to the surgery to look for other patients. Although they become disoriented in the passageway, they find a bolted door that leads to a horrific cellar guarded by an insane man named Borg. They find another two men and one woman, all insane and horribly disfigured. Just after Ramsay recognizes one of the men as Curry, Cadman, his mute servant Casimir, Daphne and Mungo confront them. They then drag Laurie and Ramsay away, unaware that the cellar keys have dropped. When Ramsay calls Cadman a madman, Cadman takes him to Angelina's room and explains that his love for her has made him do what he must to save her. Moments later, Odo arrives. When he shows Cadman Miss Daly, the doctor is furious that she is dead. Odo claims that the police delayed his administering the antidote in time, then suggests that Laurie would be a perfect substitute for the two-person operation scheduled for that night. As Daphne and Mungo drag Laurie away, Casimir signals to Cadman that the police have come. Cadman tells Odo to sneak out the back, then greets Steele. Steele enquires about Odo, but Cadman says that he left after asking for money. Meanwhile, in the operating room, Ramsay is trying to delay the operation, and when Daphne steps out, uses an anesthetized cloth to put Mungo to sleep. As Daphne enters the passageway to the study, she is confronted by Curry and Borg, who have escaped and knock her down into the hearth's fire. She then runs screaming through the house, engulfed in flames. Now Ramsay administers the antidote, but as Laurie starts to awaken, so does Mungo, who immediately attacks her. She escapes Mungo's grasp, while Curry, Borg and the others enter the room and strangle him. Just then, Cadman comes into the surgery carrying his unconscious wife. When Borg yells "Kill, Kill!" Cadman backs out and falls off the stairway landing to his death, his wife still in his arms. Now Steele and the police arrive, with the apprehended Odo and Casimir in tow. Ramsay and Laurie then leave the house as a new day dawns.