I can give you a general idea of what the plot diagram would look like. If you need more specific details, use the story to fill them in. eNotes also provides a thorough summary of the story. The format I used for this follows the "classic" literary plot diagram.
The "introduction" or "exposition:"
- This would include the introduction of discussion between Dupin, the unnamed narrator, and the Prefect of Parisian police.
- The introduction would probably also describe the main conflict of the story: that a letter has been stolen from the French Queen by a political opponent, Minister D—.
The rising action of the story:
- The Queen cannot formally accuse the thief as the letter contains private information.
- The Queen is being blackmailed.
- The police have tried several times to retrieve the letter.
- They have searched the minister's house.
- They have pretended to mug him twice, thinking he carried the letter.
- The Prefect thinks the minister is a poet, and therefore, a fool.
- A large reward has been offered for retrieval of the letter.
- The letter must always be close to the minister so he can use it any time.
- Dupin tells the Prefect to thoroughly search the man's apartment again.
- The Prefect returns one month later, with no success.
- He advises Dupin that the reward has been doubled, and the Prefect will split it with anyone who can assist him.
- Dupin tells the Prefect to write him a check on the spot, and gives the astounded Prefect the stolen letter.
- The Prefect rushes out.
- Dupin explains what he did to get the letter.
- He realized the police searched places where they would have hidden the letter.
- Dupin refers to a fable about a boy who was able to beat his opponents in guessing games by studying their behavior.
- Dupin explains that the Prefect's presumption that because the minister was a poet, that he was also a fool was a flawed conclusion: the minister is smart.
- Dupin mentions that the best way to hide something is to place it in plain sight.
- Dupin visits the minister wearing sunglasses to hide the movement of his eyes from the minister as he looks around the room.
- Dupin leaves his snuff box behind so he can retrieve it the next day.
- Dupin returns the next day to get it.
- Dupin arranges the distraction of a gun going off in the street—and to occupy the minister's attention.
- Dupin takes the "purloined" letter and leaves a substitute.
- He explains he didn't take the letter and run because he thought the minister might catch him and have him killed.
- As a supporter of the Queen, Dupin wants to help her; he also wants to witness what happens when the minister tries to blackmail the Queen again: he will be exposed.
- Dupin also alludes to a personal vendetta toward the minister for some unnamed "offense" committed against Dupin years before.
- In the replacement letter Dupin left, therein lies a clue for the minister in the form of Dupin insignia.