* The Narrator's Unreliability: The story is told from the perspective of the murderer, who is clearly deranged. He claims to be sane, but his actions and the way he describes them reveal his insanity. This creates a sense of mystery around his true motives and the validity of his claims.
* The Hidden Truth: While the reader knows the narrator has committed murder, the police remain oblivious. The mystery lies in whether they will ever discover the truth and if the narrator will ultimately be caught.
* The Psychological Puzzle: The story is less about solving a crime and more about exploring the psychological workings of a disturbed mind. The reader is left to piece together the narrator's motives, his descent into madness, and the extent of his guilt.
* The Mystery of the Heartbeat: The famous "tell-tale heart" is a symbol of guilt and the narrator's inability to escape his own conscience. This element adds a layer of mystery to the story, as we are left to ponder the meaning of the sound and its significance to the narrator's mental state.
In conclusion, "The Tell-Tale Heart" is a mystery in its exploration of the human psyche, the reliability of the narrator, and the hidden truth surrounding the murder. It's a psychological mystery that leaves the reader pondering the dark depths of the human mind.