The main idea of "The Tell-Tale Heart" is to probe the depths of the human psyche; Poe uses the story to examine the motives and pathology of a mad man, who interestingly enough is the narrator of the story and tells the reader from the very beginning:
"TRUE!—NERVOUS—VERY, very dreadfully nervous I had been and am; but why will you say that I am mad?"
The story chronicles the apparently insane, and very unreliable, narrator's plot to murder his elderly roommate. By the end of the short story, the narrator is tortured by an imagined, or possibly real, thumping; his guilty conscience leads the narrator to assume the worst--it is the old man's beating heart. Its torturous thumping condemns his actions, and the narrator, not able to stand it any longer, rips away the floor boards to reveal the evidence of his crime to the police. In this way, another main idea of the story is also guilt and how a guilty conscience can manifest itself.