Following the funeral of Simon Cordier [Vincent Price], a French magistrate and amateur sculptor, his secret diary is read aloud by his pastor friend to a circle that includes Cordier’s servants and a police captain. The diary reveals that Cordier has come into contact with a malevolent entity—the horla [Joseph Ruskin], an invisible yet corporeal being with limited psychokinesis and the power to fully seize a human’s mind. The narration hints that Cordier’s horla is part of a wider race of evil entities dedicated to driving people to madness.
Cordier’s first haunting occurs after he encounters a prisoner whom the horla has driven to murder four people. The horla possesses the inmate and tries to kill Cordier, but in self-defense Cordier ends up killing the man. The magistrate then finds himself bearing the burden of the prisoner’s troubles as the horla’s hauntings shift toward him, casting doubt on his own sanity.
As the terror tightens its grip, Cordier fears he is slipping into madness and seeks help from an alienist, who offers a practical suggestion: take up a hobby to steady the mind. He returns to his old passion for sculpture, and soon he meets a model who becomes central to his renewed life. Odette Mallotte [Nancy Kovack] is a married local woman whom Cordier believes could offer him happiness and meaning, and he pledges his devotion in turn. Yet the horla tests this bond, casting doubt on whether the model is a true partner or merely a charming but self-serving lure.
This internal conflict drives Cordier into a troubling spiral. The horla insists that the model is not what she seems—a scheming gold-digger—and compels Cordier to view her through a jaundiced, distrustful lens. The line between protection and coercion blurs, and Cordier’s judgment—once trusted as a magistrate—begins to falter under the entity’s influence.
In a violent episode born of paranoia and desperation, Cordier murders Odette with a knife. Her decapitated body is found in the river, and the crime is attributed to her husband, not Cordier, further entangling the lives around them. As danger closes in from every side, Cordier becomes convinced of the horla’s reality and resolves to take drastic action to end its menace once and for all.
He lures the horla into his house at night and, in a final confrontation, hurls an oil lamp at the curtains, setting the residence ablaze. He succeeds in destroying the horla, but he sacrifices himself in the fiery blaze that consumes the house.
The film closes with the same roundtable reunion after the diary is read. Some attendees insist that Cordier must have been mad and that the horla could not exist; others remain uncertain, wondering if the entity might have been real. The priest lends a concluding, cautionary note: wherever evil exists, the horla may exist as well.
