Diary Of A Madman is an exploitative horror film taken from the classic story by Guy de Maupassant. When the French magistrate strangles a condemned prisoner in self defense, the dead man's soul takes over his will. Simon (Vincent Price) is plagued with the murderous soul of the dead man and proceeds to engage in murderous mayhem. Knowing help is beyond human control, the sinful Simon realizes he can only escape the ghostly grasp by a baptism of fire. Simon enters the fiery flames knowing he risks his own life in his efforts to purge the evil spirit from his being.