The story is told in the first-person by an unnamed narrator who murders an old man, also unnamed. It is unclear what the relationship is between the narrator and his murder victim. It has been suggested that the murder victim is the narrator's father or that the narrator is the old man's servant. The two appear to have lived in the same house.
The narrator describes smothering the old man to death, hearing the unusually loud beating of the old man's heart while doing so, cutting up the body and hiding it under the floorboards. The narrator carefully hides any trace of the crime, however, a neighbor hears the old man scream and calls the police.
When some police officers arrive, the narrator tells them that he was the one who screamed because he was having a nightmare. He tells them that the old man is away and invites them to look around. While the police officers are inside the house the narrator starts to hear a sound which he believes is becoming increasingly louder, although the police officers can not hear it. The narrator believes that it is the old man's heart still beating under the floorboards, giving away the secret of his crime.
Unable to bear the sound any longer, the narator confesses to the murder and tells the police officers that they will find the old man's body parts beneath the floorboards.
The narrator of the tale insists that he is not mad, just very nervous.