"The Horla" is written in diary format by an unnamed narrator. At the beginning of the story, his entries seem rational and well-informed, but by the end of the story, they have become increasingly unhinged and alarming, indicating a decline in the narrator's mental state.
At the beginning of the story, the narrator is conscious that he is starting to feel unwell. He associates this mysterious illness with a "force" he feels aware of in his house, and he marvels at the depths of what we still don't know about the world. He begins to experience apparent sleep paralysis, and associated feelings of dread when he is in bed at night. Increasingly, he starts to imagine that somebody is following him when he leaves the house, although when he looks, there is nobody there. At last, he decides to go away for a rest cure to Mont St. Michel, as nothing else seems to help him.
At Mont St. Michel, he has a wonderful time, but meets a monk who tells him many local legends, most of them supernatural. The monk tells the narrator that humankind only understands a very small part of what really exists in the world, which makes the narrator pensive.
The narrator returns home, and finds that his coachman is also ill and his old nightmares have returned. He imagines that somebody is sucking his blood while he sleeps. He notes that the contents of his water bottle are being drained in the night, so he sets out some milk for the unseen being, as a test, and finds that it, too, disappears.
There is an interlude when the narrator goes to stay with his cousin, Madame Sable, and dines with her and a hypnotist, Dr. Parent. The doctor successfully hypnotizes Madame Sable, despite the fact that nobody believes in his powers; he is able to convince her to go and ask for five thousand francs from her cousin, which she does. Eventually, the doctor removes the effects of the hypnotism from Madame Sable, after which she remembers nothing. This troubles the narrator greatly, as it indicates still greater depths of what cannot be rationally understood.
Returning home, the narrator begins to imagine more and more that he sees the "invisible hand" of this force during the day, and starts to question whether he is going mad. He begins to talk of the force as "He" and then as "the Horla," stating that he feels it always near him. Eventually, he "sees" the creature as a mist in the night, and determines that he must kill it. Luring it into his room and locking it in, he then sets the house on fire.
At the end of the story, the house is ablaze, but the narrator is suddenly unsure as to whether he has actually killed the Horla—if he has not, he fears he must kill himself.
The story unfolds in a series of journal entries written by an anonymous narrator. Over four months, the narrator recounts his growing uneasiness over strange incidents occurring in his country house near Rouen, France. It is apparent that he is a man of considerable wealth. He mentions having several servants, he refers to an idyllic childhood in a large country home, and he enjoys a life of leisure throughout his narrative.
On the evening of May 8, the narrator is delighted to see a Brazilian ship sailing down the Seine. In the days that follow, however, he finds himself afflicted with a strange sense of malaise. He suffers from a slight fever and becomes increasingly depressed. He is convinced that he is facing some unknown misfortune, and his condition worsens whenever he walks along the river or as night approaches. The local doctor who cares for him cannot find a physical reason for his malady.
Soon the narrator reports that he is having nightmares. He dreams that an invisible creature approaches him as he sleeps and tries to strangle him. Each time he has this dream, he awakens in a cold sweat, only to find that he is alone and that his door is still locked. In despair, he leaves his country home and spends several days at Mont Saint-Michel. While he is there, his malady appears to vanish entirely, so...
(The entire section is 1,901 words.)