Simon Cordier, a French magistrate and amateur sculptor comes into contact with a malevolent entity. The invisible – yet corporeal – being, called a “horla” is capable of limited psychokinesis and complete mind control.

In 19th century England, a noted brain surgeon rescues a former student from being hanged on a false conviction for murder, and spirits him away to an ancient, repurposed abbey far in the countryside. There, he connives his pupil into assisting him in mapping the functions of the various parts of the human brain, using living subjects who are under a terrible animation-suspending drug called “black sleep”. Subsequently, the student, along with the daughter of one of the subjects, discover that most of these subjects have survived but are being kept in a dungeon-like cellar, in various stages of derangement…

Two identical twin sisters…one is very,very good–one is very, very bad. The good twin is due some big bucks. The bad twin wants ’em. Since they look just alike…look out.