The narrator of "The Tell-Tale Heart" has had a nervous breakdown which has left him slightly deranged. He is hypersensitive to benign aspects of his environment and becomes fixated on the things that upset him. Edgar Allen Poe emphasizes the narrator's dread and anxiety by dilating his sense of time at key points, so that his moments of suffering pass slowly and the reader experiences them at that pace.
Although the narrator cares for his elderly neighbor and has always considered him a friend, the sight of the old man's cloudy eye begins to agitate him. Eventually he can no longer bear the disgust he feels when he looks at the old man and decides that he must kill him.
He plans the murder carefully and practices entering his neighbor's room silently at night so that when the time comes, the old man will have no warning or chance to defend himself. The narrator describes how stealthy he must be:
And every night, about midnight, I turned the latch of his door and opened it—oh so gently! [...]—very, very slowly, so that I might not disturb the old man's sleep. It took me an hour to place my whole head within the opening so far that I could see him as he lay upon his bed.
On the eighth night of creeping into his neighbor's room, the narrator accidentally awakens him and is forced to freeze in position waiting for him to fall back asleep:
I kept quite still and said nothing. For a whole hour I did not move a muscle, and in the meantime I did not hear him lie down.
The narrator finally opens his lantern just a crack to look at the old man, and the light falls directly on the old man's blind eye, filling the narrator with rage and disgust. He stares at the grotesque eye across the room and fancies he can hear the old man's heart beating rapidly with fear. The sound begins to upset and even infuriate the narrator as he fixates upon it. At last, he flings himself at the old man and smothers him under his mattress just to still the awful sound of his heartbeat.
The narrator disposes of the corpse beneath the floorboards and is congratulating himself on his cleverness when the police come knocking. They have been called to investigate a disturbance at the property. The narrator is confident that no evidence of his crime remains, so he invites the officers in and chats with them. They are satisfied that everything looks normal, but rather than leave, they sit with the narrator and continue chatting with him. The narrator is conscious of the old man's corpse beneath the floor of the room in which they sit, and he grows anxious for the officers to depart.
... ere long, I felt myself getting pale and wished them gone. My head ached, and I fancied a ringing in my ears: but still they sat and still chatted.
The ringing in the narrator's ears grows louder and louder until it seems to resemble the old man's heartbeat. In mounting horror, the narrator becomes convinced that the officers can hear it too and that they are aware of his crime and are merely taunting him by pretending to be friendly. When his anxiety reaches fever pitch, he leaps to his feet and screams his confession.