The narrator is bothered by the old man's eye. He loved the old man and never had any problems with him, but the old man's eye was too much for the narrator to handle. So, he is very nice to the old man a week before his murder.
The narrator goes to the old man's house every night for seven days to kill the old man, yet he didn't because the man's eye was always shut while he slept. In his mind, if the eye was shut, it didn't bother him.
On the eighth night, the old man wakes up, and the narrator is waiting for him to go back to sleep. Eventually, the narrator shines the slightest bit of light on the eye, and sees that it was open.
The narrator begins to hear the sound of the old man's heart beating, and he begins to get furious. He leaps into the room and suffocates the old man to death with his mattress.
The narrator hides the old man's dead body under the floor boards. Then police come in to talk about a neighbor reporting hearing a scream. He talks and acts so confident that he sits in the very room where the old man's body is hidden.
The narrator, sitting on top of the floorboards where the old man is, kept hearing the heartbeat of the old man. He grows impatient and can't handle the sound of the old man's heartbeat. He then confesses and turns himself into the police.