Before beginning his account, the unnamed narrator claims that he is nervous and oversensitive but not mad, and offers his calmness in the narration as proof of his sanity. He then explains how although he loved a certain old man who had never done him wrong and desired none of his money, the narrator could not stand the sight of the old man's pale, filmy blue eye. The narrator claims that he was so afraid of the eye, which reminds him of a vulture's, that he decided to kill the man so he would no longer have to see it.
Although the narrator is aware that this rationalization seems to indicate his insanity, he explains that he cannot be mad because instead of being foolish about his desires, he went about murdering the old man with "caution" and "foresight." In the week before the murder, the narrator is very kind to the old man, and every night around midnight, he sneaks into the old man's room and cautiously shines a lantern onto the man's eye. However, because the eye is always closed and the narrator wishes to rid himself of the eye rather than the man, the narrator never tries to kill him, and the next morning, he again enters the chamber and cheerfully asks how the old man has slept, in order to avoid suspicion.
On the eighth night, the narrator is particularly careful while opening the door, but this time, his thumb slips on the lantern's fastening, waking the old man. The narrator freezes, but even after an hour, the old man does not return to sleep because he feels afraid and senses someone's presence. At length, the narrator decides to slowly open the lantern until the light shines on the old man's eye, which is wide open. The narrator's nerves are wracked by the sight, and he fancies that because of his oversensitivity, he has begun to hear the beating of the old man's heart.
The beating firms his resolve as he continues to increase the intensity of the light on the man's eye. The beating grows louder and louder until the narrator begins to worry that a neighbor will hear the noise, so he decides to attack. The old man screams once before the narrator drags him to the floor and stifles him with the mattress. When the narrator stops hearing the beating, he examines the corpse before dismembering it and concealing it beneath the floorboards. He laughs somewhat hysterically as he describes how the tub caught all the blood, leaving no stains on the floor.
By the time he finishes the clean-up, it is four in the morning, and someone knocks on the door. In a cheerful mood, the narrator answers the door only to find three policemen who have come to investigate because a neighbor heard the old man's shriek and alerted the police to the possibility of foul play. The narrator invites them inside, knowing that he has nothing to fear, and he explains that he had been the one to yell as a result of a bad dream and that the old man is currently out visiting the country. He shows the policemen the house and confidently allows them to search it before bringing out chairs which he, in his assurance, places on top of the floorboards that hide the corpse.
The narrator's lack of suspicious behavior convinces the policemen that nothing is wrong, and they sit down on the chairs and chat with him. However, after a while, the narrator begins to wish that the policemen would leave, as his head aches and he hears a ringing in his ears. The ringing increases in volume, for which the narrator compensates by chatting more jovially, but it finally turns into a dull beating which also begins to rise in volume. The narrator becomes more and more agitated in his behavior, gesturing wildly and pacing back and forth, but the policemen hear and suspect nothing.
Soon, the narrator begins to suspect that the pleasantries of the policemen are merely a ruse to ridicule his distress. However, he cannot stand the intensity of the beating and grows tired of what he perceives as the mockery of the policemen. He feels that he "must scream or die," so he finally shrieks the truth, telling the policemen to tear up the floorboards and reveal the beating of the old man's heart.
The protagonist of the "The Tell-Tale Heart" is a classic example of Poe's unreliable narrator, a man who cannot be trusted to tell the objective truth of what is occurring. His unreliability becomes immediately evident in the first paragraph of the story, when he insists on his clarity of mind and attributes any signs of madness to his nervousness and oversensitivity, particularly in the area of hearing. However, as soon as he finishes his declaration of sanity, he offers an account that has a series of apparent logical gaps that can only be explained by insanity. In his writings, Poe often sought to capture the state of mind of psychotic characters, and the narrator of this story exhibits leaps of reasoning that more resemble the logic of dreams than they do the thought processes of a normal human being.
The narrator's emotional instability provides a clear counterargument to his assertions of good judgment. In almost no cases does he respond in the manner that one would expect. He is so bothered by the old man's vulture-like eye that his loathing overcomes his love for the man, leading him to premeditate a murder. Later, when he finally succeeds in killing the victim, he becomes positively cheerful, feeling that he has accomplished his goal cleverly and with the rationality that he associates with sanity. However, the unsuspecting behavior of the policemen suggests that the narrator has become essentially unaware of his behavior and his surroundings. Because he cannot maintain the distance between reality and his inner thoughts, he mistakes his mental agitation for physical agitation and misinterprets the innocent chatter of the policemen for malevolence. Nevertheless, he imagines the whole time that he has correctly and rationally interpreted all the events of the story, suggesting that in Poe's mind, the key to irrationality is the belief in one's rationality.
The irony of the narrator's account in "The Tell-Tale Heart" is that although he proclaims himself to be too calm to be a madman, he is defeated by a noise that may be interpreted as the beating of his own heart. Because of the unreliability of the narrator, it is impossible to know for certain if the beating is a supernatural effect, the product of his own imagination, or an actual sound. However, a likely logical explanation is that when the protagonist is under stress, he hears the sound of his heart, "a low, dull, quick sound, such as a watch makes enveloped in cotton," and he mistakes it for the sound of the old man's heart. This lack of understanding parallels his lack of awareness of his actions as he chats with the policemen and highlights the lapses in reason which belie his claims of sanity.
In order to create a narrative which will convince the reader of the protagonist's instability, Poe uses vocabulary that is consistently ironic or otherwise jarring to provoke a reaction contrary to that which the narrator desires. The rhetorical technique that he uses in his account is to manipulate the connotations of words, but he is never subtle enough to hide his attempt to spin the argument. Where an outside observer might describe him as having plotted to observe the old man as he sleeps, the narrator tells the reader that "you should have seen how wisely I proceeded--with what caution--with what foresight--with what dissimulation I went to work!" By exploiting his choice of words such as "wisely" and "caution," he seeks to deceive the reader and explain his actions as those of a prudent, clever individual. However, the blatancy of his attempt at deception enlightens rather than hoodwinks his audience.
Much as the minute depiction of the prisoner's experiences and senses creates an atmosphere of anticipatory terror in "The Pit and the Pendulum," Poe's manner of describing sound becomes a particularly important vehicle for conveying the mood of "The Tell-Tale Heart." His description of the sound in the last few paragraphs of the tale is marked by repetitions that are clearly intended to imply the crescendo of noise. When he says, "The ringing became more distinct:--It continued and became more distinct," we sense the building tension. The increasing intensity of the beating is again emphasized by the three repetitions of the phrase "but the noise steadily increased." Finally, as the narrator's sentences turn rapidly into exclamations, his repetition of the word "louder" echoes the sound of the beating heart, and his final shrieks shatter the tension with his confession.