The narrator is sitting and thinking about killing an old man because of his eye. The man goes in the old mans room every night for 8 days planning what and how he's going to kill the old man. The man has a lantern to shine on his eye at night too.
The narrator on the night he was going to kill the man was about to open the lantern when his finger slipped and woke up the old man. The old man though couldn't see the narrator because it was so dark in his room. After an hour the old man was still awake.
The narrator could hear the old man's heart beat and got worried because he thought the neighbors could hear it too. The man shrieked when he saw him, so the narrator dragged the man off the bed and put the bed over him and killed the old man.
The neighbors heard the shriek so they called the police. The police came and searched the house and didn't find the man because he was underneath board planks. The officers and the man talked and the narrator started hearing a heart beat like the one he heard when the old man was alive.
The heart beat got louder and louder, and the cops still didn't hear the heart beat. After a couple minutes he couldn't handle it any longer and said, "Villains! Dissemble no more! I admit the deed! -tear up the planks!-here, here!-it is the beating of his hideous heart!"