The narrator is haunted by the old man's eye. The eye drives him into insanity, and he decides to kill the old man and rid him of the horrid eye.
For seven days the narrator sneaks into the old man's room. He slightly opens his lantern so a small beam lands on the old man's eye. The eye is always closed so he can not kill him, for it is not the old man he wants to rid himself of, but his eye.
On the eighth night the narrator makes a noise that wakes the old man. The old man's heart starts pounding from the fear of someone being in his house. The old man's heart starts beating so loud the narrator thinks a neighbor will hear, so in fear of someone hearing, he jumps out to kill the old man. The old man shrieks once before the narrator can kill him. The narrator disembodies the old man and hides him under the floorboards.
Three officers show up telling the narrator that a neighbor had called saying they had heard a scream in the night. The officers search the house and find nothing. The narrator handles this very well. The officers become comfortable and start chatting.
The narrator hears the old mans heart beating under the floorboards. The noise drives him to confess his crime to the officers.
Here, here! It is the beating of his hideous heart!