The narrator is stalking the old man who sleeps, furthermore the narrator hates the old man's eye. The narrator goes to great lengths to make sure that he isn't seen by the old man. THe narrator hates his "vulture like eye" so much he plans to murder him to be rid of the "evil eye".
*Bump Bump Ba-Badump*
However on the 8th night of the narrator stalking the old man wakes, perhaps because the narrator accidentally let out a small and faint chuckle...The old man denies and pretends it is nothing but his heart starts beating, and it gets louder and louder.
HERE LIES THE OLD MAN!!!
The heart beat kept getting louder! The narrator became a bit anxious, so then he striked! The narrator dragged him out of his bed and put the heavy bed over the old man, thus killing him.
The narrator skillfully hid the body under wooden planks, and cleaned up all of the evidence. However three officers came to the door because someone heard a shriek. The narrator got cocky and let them in for a rest and convinced the, that it was him who had shrieked from a night terror. However, the narrator kept hearing the old mans heart beat.
However, from the narrator's insanity he kept hearing the old mans heart beat louder and louder, thus bringing him to reveal to the officers that he indeed did kill the old man.