When Judge Simon Cordier (Vincent Price) goes to see Louis Girot (Harvey Stephens), a prisoner he sentenced to death, the criminal insists he is not guilty and that he is inhabited by an evil spirit. Cordier dismisses Girot's claims, but he becomes unnerved when Girot lights himself on fire to supposedly end the demonic possession. After the visit, Cordier, beset by insomnia, begins to have odd visions and starts to wonder if Girot might have been telling the truth after all.