England, 1872. The night before he is to be hanged for a murder he did not commit, young Dr. Gordon Ramsey is visited in his cell by his old mentor, eminent surgeon Sir Joel Cadmund. Cadmund offers to see that Ramsey gets a proper burial and gives him a sleeping powder to get him through the night, which Ramsey takes, unaware it is really an East Indian drug, "nind andhera" ("the black sleep"), which induces a deathlike state of anesthesia. Pronounced dead in his cell, he is turned over to Cadmund, who promptly revives him and takes him to his home in a remote abbey. Cadmund explains he believes Ramsey is innocent and needs his talents to help him in an project, which he is reluctant to immediately discuss further. In fact, Cadmund's wife lies in a coma from a deep-seated brain tumor, and he is attempting to find a safe surgical route to its site by experimenting on the brains of others, whom Ramsey comes to learn are alive during the process, anesthetized by the "black sleep", and are taken to a hidden recovery room in the abbey from which few emerge, though they still live.