In a London prison in 1872, Dr. Gordon Ramsay is awaiting execution for the murder of a man named Curry, despite his claims of innocence. He is visited by renowned surgeon Sir Joel Cadman, who offers him a chance to save his life in exchange for assisting Cadman with experiments at his estate. Cadman gives Ramsay a potion that he calls "The Black Sleep," which induces a deathlike state that can lead to actual death if an antidote is not administered in time. The next morning, Ramsay is discovered in his cell, apparently dead, and Cadman takes the body, supposedly for burial, with his assistant Odo. When Ramsay is revived at Cadman's estate, he is startled by screams from a young woman named Laurie, who is being attacked by a large man named Mungo. The only person who can control Mungo is Cadman's nurse, Daphne, who quiets the attacker and leads him away. Later, Cadman and Daphne visit the bedroom of Cadman's wife Angelina, who is comatose from an inoperable
brain tumor. Cadman vows that he will find a way to operate on Angelina and save her life. However, when he next meets with Ramsay, Cadman tells him that he is conducting experiments on human brains to help restore creatures like Mungo and his mute servant Casimir to normal condition. Mungo, it turns out, was actually a Doctor Monroe, one of Ramsay's former teachers, whom Cadman claims he is trying to help through his research. In a hidden laboratory/operating room in the manor, Ramsay observes Cadman's experiment on a man's brain, with Lucy and Daphne assisting. He is taken aback, though, when he sees
cerebral fluid leaking from the subject's exposed brain, indicating that he is still alive. Cadman regards potential brain injury as a necessary risk for the greater good that his experiments will produce, testing different regions of the brain to map their functions. That night, though, Laurie tells Ramsay that she thinks she can trust him and that she is actually Dr. Monroe's daughter. Her father's current state as "Mungo" was caused by an operation by Cadman. When confronted by Ramsay, Cadman admits that he was responsible and says that he is working to reverse and atone for his mistakes, but he does not mention his wife and her condition. Back in London, Odo has slipped the Black Sleep potion to a woman who knows the truth about the man Curry whom Ramsay was alleged to have killed. When the police arrive looking for her, Odo claims not to know where she is and disposes of the antidote that could revive her. At Cadman's manor, Ramsay and Laurie have concluded that Cadman's last subject was the still-living Curry and seek further evidence. In a hidden dungeon, they find locked cells with other living subjects of Cadman's experiments, all now mad and disfigured. Cadman, Daphne, Mungo, and Casimir capture Ramsay and Laurie but drop the cell keys as they leave. Cadman reveals Angelina's condition to Ramsay. He had expected to conduct an operation that required a second woman subject, but when he learns that Odo had let the intended victim die, decides to use Laurie instead at Odo's suggestion. Cadman is interrupted, though, by the arrival of police detectives looking for Odo. Meanwhile, Ramsay tries to revive the anesthetized Laurie and manages to drug Mungo while Daphne is out of the room. The nurse herself is confronted by Cadman's victims, who have escaped from the dungeon with the dropped keys. Led by a subject who believes he is
Bohemund the Crusader, they cast Daphne into the fireplace, from which she rises, aflame and screaming. Ramsay revives Laurie but Mungo awakens at the same time and attacks her. He is stopped, though, by the surging group of victims who then turn on Cadman as he enters the room with his comatose wife. Backing away, Cadman falls over stair rail and plunges to his death with Angelina. The police finally arrive with Odo and Casimir in custody, and Ramsay and Laurie depart as a new day breaks. ==Cast==