Edgar Allan Poe's short story, "The Tell-Tale Heart," is about a man (the narrator) who is very put off by the eye of the old man he lives with. Given his utter hatred of the old man's eye, the narrator plots murder against the old man.
Night after night, the narrator visits the old man's room to see if the time is right to take the old man's life. One night, the time is right. The narrator flings himself upon the old man and murders him.
After murdering the old man, the narrator places his body beneath the floorboards of their shared apartment. Soon after, the police arrive. It seems that neighbors have called; they heard loud noises. The narrator welcomes the police into the apartment, even placing his chair upon the floorboards covering the body.
Eventually, the narrator begins to hear a heartbeat. Fearful that the heartbeat is heard by the police, the narrator points out the floorboards revealing the murder.
Throughout the story, while speaking to what must be a doctor or interrogator, the narrator constantly mentions that he is not mad. This, for some readers, forces them to believe the exact opposite.