Following the funeral of Simon Cordier (Vincent Price), a French magistrate and amateur sculptor, his secret diary is read out by Simon's pastor friend to a group of people gathered around the table, Simon's servants, and a police captain. The diary reveals that Simon has come into contact with a malevolent entity. The invisible yet corporeal being, called a ''horla'' is capable of limited psychokinesis and complete mind control. It implied that Cordier's particular horla is one of a whole race of evil beings which devote themselves to driving humans insane.