"The Tell-Tale Heart" is a first-person narrative of an unnamed narrator, who insists he or she is sane but is suffering from a disease (nervousness) which causes "over-acuteness of the senses". Due to the ambiguity surrounding the identity of the story's narrator, the narrator's gender is uncertain.

The old man with whom the narrator lives has a clouded, pale, blue "vulture-like" eye, which distresses the narrator so much that they plot to murder the old man, despite also insisting that they love the old man. The narrator insists that their careful precision in committing the murder proves that the narrator cannot possibly be insane. For seven nights, the narrator opens the door of the old man's room in order to shine a sliver of light onto the "evil eye". However, the old man's vulture-eye is always closed, making it impossible to "do the work".

On the eighth night, the old man awakens after the narrator's hand slips and makes a noise, interrupting the narrator's nightly ritual. But the narrator does not draw back and, after some time, decides to open the lantern. A single thin ray of light shines out and lands precisely on the "evil eye", revealing that it is wide open. Hearing the old man's heart beating loudly and dangerously fast from terror, the narrator decides to strike, jumping out with a loud yell and smothering the old man with his own bed. The narrator then dismembers the body and conceals the pieces under the floorboards, and ensures the concealment of all signs of the crime. Even so, the old man's scream during the night causes a neighbor to report to the police, who the narrator invites in to look around. The narrator claims that the scream heard was the narrator's own in a nightmare and that the man is absent in the country. Confident that they will not find any evidence of the murder, the narrator brings chairs for them and they sit in the old man's room, on the very spot where the body is concealed, and suspect nothing, as the narrator has a pleasant and easy manner.

The narrator begins to feel uncomfortable and notices a ringing in the narrator's ears. As the ringing grows louder, the narrator comes to the conclusion that it is the heartbeat of the old man coming from under the floorboards. The sound increases steadily, though the officers seem to pay no attention to it. Terrified by the violent beating of the heart, and convinced that the officers are aware not only of the heartbeat, but also of the narrator's guilt, the narrator breaks down and confesses, telling them to tear up the floorboards to reveal the remains of the old man's body.