This story starts off without hesitation: a nameless person explains that he is and was extremely nervous, but is not and was not insane. Instead of being nuttier than a fruitcake, the narrator claims he has a "disease" which makes all his senses—especially his hearing—super-sensitive. To further prove that he isn't insane, the narrator shares an event from his past.
Let's jump into his tale:
The narrator has an idea that he can't shake: he's deeply, deeply disturbed by his landlord (or roommate—the narrator doesn't make his relationship with the guy super-clear.) He loves the old man and has nothing against him—and he doesn't want his money, either. But the old man has a really creepy eye, which is "pale blue […] with a film over it" (2). The narrator hates the eye and decides to kill the old man to be free of it.
Because that sounds sane, right?
To that end, the narrator goes to the old man's room every night at midnight, for seven days. Each night the narrator opens the man's door and puts in a lantern (it's a special "dark lantern," with panels that can be adjusted to release individual rays of light). After the lantern, the narrator puts his head through the doorway, extremely slowly, and then opens the lantern so a tiny beam of light shines on the old man's eye. Each night the old man doesn't open his eye, so the narrator feels that he can't kill him. Remember: he doesn't hate the man, he hates the man's eye.
On the eighth night, the old man hears the narrator at the door and wakes up. The narrator hangs out there in the dark for a long time, motionless and then, with a scream, plunges into the totally dark room, opening the lantern, and shining light on the old man's eye. The narrator drags the old man, who has only screamed once, off the bed, and then pulls the bed on top of the man. When the narrator hears the man's heart stop beating, he removes the bed and checks to make sure the old man is really dead.
So the narrator—who is still telling the reader that he's 100% sane—cuts him up and hides his remains under the floor. More proof of the this guy's sanity? He cut the old man up in the bathtub, so there aren't any bloodstains on the floor.
Then three policemen come a'knocking, because a neighbor had heard the old man's scream and called them. The narrator says he screamed while sleeping, and claims that the old man is out of town.
After convincing the cops nothing fishy happening, the narrator brings them into the old man's bedroom and they all sit down to chat. While the'are all shooting the breeze, the narrator starts hearing a terrible ticking noise, which gets louder and louder until the narrator freaks out, confesses, and points the police to the old man's body, stating that the sound is coming from the old man's heart.
Yup: our narrator is definitely not the sanest man who ever lived.