With the immediate and sudden introduction of the unreliable narrator, Poe establishes a mood of psychological horror. For, much as he does in his other story, "The Cask of Amontillado," Poe subverts the Gothic conventions of mysterious happenings and supernatural beings; instead, the real horror lies in the capabilities and perceptions of the narrator himself as suggested in Poe's opening line.
This narrator who commits the horrific deeds of murdering the old man whom he insists that he loves and dismembers the corpse seeks to explain his terrible actions as merely the result of being "nervous." Yet, he complains of "uncontrollable terror" while, at the same time, feeling "a wild audacity of my perfect triumph" as he buries the old man undetected. Finally, the ringing in his ears and the "low, dull sound" of the dead man's heart that he nervously imagines he hears creates the real terror as the narrator does, indeed, go mad.
TRUE! --nervous --very, very dreadfully nervous I had been and am; but why will you say that I am mad?
The opening line of Edgar Allan Poe's classic short story, "The Tell-Tale Heart" sets the mood for the story at once. Using the "nervous" voice of the narrator, who may or may not be mad, immediately establishes a mood of paranoia. The mood of a story results from details, descriptions, imagery, or figurative language that the author provides through the setting, characters, or even dialogue. In "The Tell-Tale Heart," Poe utilizes the uneasy voice of the narrator to establish the mood for the reader. The narrator's emphasis on being "nervous--very, very dreadfully nervous" imbues the exposition of the story with feelings of uncertainty.