The narrator of the story asks his friend Moxon if he seriously believes that machines can think. Moxon questions what he means by "machines," arguing that by most definitions of the term, human beings would be considered machines as well. The narrator irritably says that a machine cannot be a man, but rather something that man controls. Moxon comments that this is only when it does not control him.
The narrator knows that Moxon spends a great deal of time working in his machine shop and often suffers from insomnia. He wonders if Moxoon is beginning to go insane.
The narrator asks what Moxon believes machines think with in the absence of a brain, and Moxon instead asks him what plants think with. Moxon points out that many plants seem to show evidence of thought - for example, a vine will crawl towards a post and shift direction if the post is moved. Moxon goes as far to say that this proves that plants can think. He further says that this explains the phenomena of crystallization; just as soldiers form lines, so do the atoms of materials rearrange themselves.
The two hear a thumping coming from Moxon's machine shop. Moxon looks agitated, and runs into the shop. The narrator hears the sounds of a struggle, and when Moxon returns, he has four scratch marks on his face. Moxon continues to explain his odd philosophy that all matter is aware and thinking. He brings up Herbert Spencer's definition of life, which is "a definite combination of heterogenous changes, both simultaneous and successive, in correspondence with external coexistences and sequences." Moxon points out that the action of a machine is included in this definition.
The narrator asks who is in Moxon's shop right now, and Moxon says that it is only a machine that he accidentally left on. Moxon goes on to continue his philosophical musings, but the narrator becomes frustrated and storms out of the house.
The narrator puzzles upon something Moxon said - "Consciousness is the creature of Rhythm" - as he walks through the rainy night. He realizes how deep and insightful this observation really is, and runs back to Moxon's house in order to discuss this topic with him further.
He finds Moxon in the machine shop, sitting across from a figure who has his back to the narrator. The narrator pauses to watch them unobserved. The mysterious figure is short but has very thick shoulders and wears a fez. The two are playing chess. Moxon seems frantic, but the mysterious figure moves more slowly. The narrator realizes that this must be a chess-playing machine that Moxon has created. Both Moxon and the narrator are startled when the thing strikes the table with a hand.
At last, Moxon declares checkmate, and with a whirring of wheels, the machine charges at him, knocking over the candle and plunging the room into darkness. The narrator can hear the terrible sound of Moxon being choked to death by the machine. Suddenly, a blinding flash of light reveals the dying Moxon and the blank painted face of the machine. The narrator falls unconscious.
He wakes in a hospital with Haley, Moxon's assistant, at his side. Haley explains that lightning struck Moxon's house causing a massive fire, and he was buried yesterday. Haley found the narrator and carried him from the ruin. The narrator asks if Haley also rescued the chess-playing machine that murdered its maker, and Haley only asks if he really knows that. The narrator finds that he is not sure.
Moxon's belief that intelligence is expressed in all matter is called panpsychism. This philosophy, which dates back to ancient Greek philosopher Thales and has been expressed by modern thinkers such as William James, holds that mind/soul is a universal feature of all things, animate and inanimate.
Haley's appearance at the house is sudden and unexplained - he was not present during the discussion between the narrator and Moxon, and suddenly appears when the narrator is in need of saving. This has led some scholars, including Daniel Canty, to suggest that Haley is in fact disguised as the automaton. In this interpretation, the chess game between Moxon and Haley/the machine is staged for the benefit of the narrator, and after murdering Moxon, Haley picks up the unconscious body of the narrator and carries him to safety.
The machine is described as wearing a fez, suggesting an exotic Eastern costume. This may be a reference to Edgar Allen Poe's 1936 essay "Maelzel's Chess-Player," in which he debates whether a machine that has the ability to play chess may also be said to have a mind of its own. The appearance of the machine in Poe's essay - it is called the Turk - exactly parallels that of Moxon's machine. Significantly, much of Poe's argument turns on the fact that the machine sometimes loses chess games.
The fact that the house is destroyed by lightening suggests an element of divine retribution - for his attempt to usurp the creative powers of God, Moxon's house is destroyed by a thunderbolt and he is killed. As Daniel Canty puts it, the lightning is punishment for Moxon's "folly in pretending that creation was nothing but an enormous machine."