The plot of "The Tell-Tale Heart," by Edgar Allan Poe, is about the narrator's insanity and paranoia surrounding an old man who lives with him. Later in the story, the narrator's mental deficiencies worsen after he kills the old man.
According to GradeSaver.com, the narrator of "The Tell-Tale Heart" begins the story by telling his audience that he is not insane, just nervous. He continues with a calm justification as to why he killed the old man who lived with him.
In life, the old man never harms the narrator, but the old man has a glassy, blue eye that the narrator cannot stand. Every night, the narrator enters the old man's room and watches him while he sleeps. Strangely, every morning the narrator is cordial with the old man and behaves as if nothing is wrong.
On the eighth night of watching, the narrator makes too much noise and awakens the old man. The narrator waits, but the old man does not fall back to sleep because he feels someone outside the door. The narrator grows impatient and brightens his lantern to see into the room. A sliver of light filters through the crack in the door and falls directly on the old man's wide-open blind eye. The narrator is startled by the sight of the eyeball and begins to hear a thumping sound, like a heartbeat. The narrator loses control and attacks the old man, killing him. The old man yells out once before he dies. The narrator cuts the body into multiple pieces and hides them under the floorboards.
The neighbors report the old man’s cry to the police, and the officers arrive soon after to search the narrator's house. The narrator keeps a calm demeanor during the investigation and even brings the policemen into the old man's room to sit and chat casually. However, the narrator begins to hear the heartbeat again. The narrator's paranoia mounts as the heartbeat gets louder. He begins to think the only reason the policemen are being nonchalant is because they can hear the heartbeat too and know what the narrator did. Finally, the narrator's paranoia gets the best of him, and he confesses to the murder.