Reprising their roles from "The Murders in the Rue Morgue," C. Auguste Dupin and his friend the unnamed narrator appear in a small library room in Paris, silently smoking and, in the case of the narrator, contemplating two of Dupin's previous cases involving the Rue Morgue murders and the death of Marie Rôget. Monsieur G., the Prefect of the Parisian police, enters the apartment to ask Dupin's opinion of a case, although he refuses to do so in the dark because the idea is "beyond his comprehension" and thus an "oddity." He describes the case as simple but puzzling, but ignores Dupin's suggestion that perhaps its simplicity and self-evidence is what confuses the police.
According to G., a letter has been stolen from the royal apartments that the police know the thief will use for blackmail. The letter belongs to a lady who was forced to hastily place it on a table when the person from whom she wished to conceal the secret entered the room. The Minister D., who also entered, saw and interpreted the contents of the letter correctly. He then placed a letter of similar appearance beside it before retrieving the incorrect paper prior to leaving. The lady saw the substitution but was unable to point it out because of the presence of the third person, who noticed nothing. Since then, D. has used his possession of the letter for political blackmail, and because the lady is unable to publicly reclaim the letter, she has asked the police to retrieve it for her.
The narrator notes that the minister must still have the letter, since to relinquish it would be to lose his power of blackmail, but the police have been unable to locate it, despite having thoroughly searched D.'s apartment. D. cannot be keeping the letter on his person, since the police have already searched him twice. Dupin remarks that the minister cannot be much of a fool, although the Prefect disparages the man for being a poet and therefore, in the Prefect's view, unintelligent. The narrator asks the Prefect about the police's method of search, and the Prefect explains how thoroughly they have searched the apartment, particularly since the reward for the retrieval of the letter is so great. The narrator agrees with the Prefect that the letter must not be in the apartment, but Dupin asks G. to search it again before asking for a complete description of the letter.
A month later the Prefect returns, having found nothing on a second search, and mentions that he will offer a reward of fifty thousand francs, since the retrieval of the letter has become increasingly important. Dupin tells the Prefect to write the check; the astonished Prefect does so, takes the letter from Dupin, and rushes away from the apartment. Dupin explains to the narrator that the police were very skilled but that the case was not suited to the unimaginative. He provides the example of a schoolboy who was particularly skilled at a guessing game in which he was to guess whether his opponent had an odd or even number of marbles and in which he bet one marble per game. The schoolboy won because he was able to emulate his opponent's logic by imitating the other boy's face in order to see how the expression made him think. The police only think about what they believe to be the best course and fail to consider the thoughts of the Minister.
Dupin notes that the Prefect believes that D. is a fool. However, D. is also a mathematician and can thus combine creativity and logic. According to Dupin, while normal mathematicians lack imagination and would have hidden the letter away in exactly the type of place where a policeman would search, the Minister foresaw the probable avenue of investigation and chose an alternate route. Dupin offers the example of a game in which one attempts to guess the point on a globe of which the other is thinking. A novice will choose an obscure name, but a skilled player will choose a very prominent name, knowing that the other person will discard such names as possibilities because they are too obvious. The Prefect does not understand this reasoning, but Dupin places himself into the mind of the Minister and realizes that the Minister would have decided to hide the letter in the most obvious place possible.
After coming to this conclusion about the letter, Dupin visits D.'s apartment while wearing green glasses that conceal the fact that he is looking around the apartment. At length, he discovers several visiting cards and a letter that has been torn and altered in appearance hanging carelessly from a rack on the mantelpiece. D., it appears, placed the letter in full view after turning it inside out, readdressing it, and making it appear useless. Dupin memorizes the appearance of the letter while talking with the Minister and leaves a gold snuff box at the apartment. The next morning, he comes back on the pretense of having forgotten his snuff box, and when D. rushes to his window to observe a disturbance involving gunshots that Dupin previously orchestrated, Dupin substitutes the letter with a fake that he created the night before and soon returns home.
The narrator asks why Dupin did not simply steal the letter. Dupin answers that D. might have been desperate enough to have his attendants kill Dupin. In addition, he notes that after a year and a half of being subjected to the Minister's blackmail, the lady will now have the upper hand. He predicts that D. will soon embarrass himself and cause his political downfall, but he has no pity for the man because D. is "an unprincipled man of genius" who once did Dupin a wrong, which Dupin good-naturedly promised to return. Dupin admits that he would like to know the man's thoughts when he opens the letter to read a quote from Crebillon's Atrée et Thyeste which translates to "If such a grievous plan is not worthy of Atreus, then it is worthy of Thyestes." Dupin knows that D. will recognize Dupin as having gotten his revenge.
Whereas Dupin's investigation in "The Murders in the Rue Morgue" established the basic form for a classic whodunit mystery, "The Purloined Letter" takes an entirely different route to highlight Dupin's methods of ratiocination and use of creativity to place himself in the mind of the criminal. The case is clear in that the thief and the details of the crime are perfectly obvious, but what is not clear is how to outwit the thief and return the letter to its rightful owner. The story shows much more of the character of the Prefect, who merely appeared in order to act disgruntled and embarrassed at the end of the first Dupin story. As a result, the narrative includes two characters, the narrator and the Prefect, who serve as obvious foils to Dupin, while the Minister's similarities to Dupin advance the concept of double selves that is prevalent in so many of Poe's stories.
With his energy, obvious emotions, and lack of insight, the Prefect stands in direct opposition to Dupin's calmer, more analytical approach to solving cases. His major fault is that he does not understand that the key to solving a case is to think in a way that successfully approximates the mindset of the criminal; instead, he resorts to trying to find more and more clever ways that he would personally have chosen to hide the letter while chasing answers that are increasingly further away from the correct solution. Whether the case is grisly and bizarre as in "The Murders in the Rue Morgue" or simple and clever as in this instance, Monsieur G. requires the assistance of Dupin because of his consistent inability to imagine the psyches of others. The narrator is less removed from Dupin's point of view and is more inclined to think as Dupin would, but he lacks the perception that allows him to reason out the case himself and becomes a surrogate for the reader. Because the narrator writes in the first person, he takes on the role of conveying and interpreting Dupin's brilliance for the average individual.
The clash between the Prefect and Dupin is revealing of their opposing temperaments, but it is also a source of humor, as Dupin constantly but subtly takes ironic verbal jabs at the oblivious Prefect, whom the story constantly shows at a relative mental disadvantage. When the Prefect explains that the owner of the letter contacted the Parisian police to help her retrieve the letter, for example, Dupin sarcastically remarks that it must be a reflection of the Prefect's intelligence, a prod which the latter fails to notice, therefore highlighting his inability to understand anyone's thoughts but his own. Later, the Prefect dismisses the Minister because he is a poet and thus a fool, but Dupin notes drolly that he too is something of a poet. The exchange is entertaining because the Prefect is totally unaware of the fact that a poet's creativity is the trait that allows one to think like a Dupin or a Minister D. instead of like the Prefect.
On the other side of the divide between the unimaginative and the analytical lies Minister D., who might be Dupin's equal in understanding the human mind. The concept of alter egos often appears in Poe's short stories, and Minister D. functions as the criminal version of Dupin, a man who generally acts on the side of the law. Dupin evidently recognizes the similarity, for he tells the narrator that the Minister "is that monstrum horrendum, an unprincipled man of genius," and he takes pleasure in trumping the Minister in a battle of wits. In the fake letter that Dupin leaves for the Minister, he provides a quote about two Greek brothers from mythology, Atreus and Thyestes. Thyestes commits adultery with Atreus's wife, and in revenge, Atreus kills and cooks Thyestes's sons before feeding them to his brother. The quote implies that although Atreus committed a great wrong, Thyestes was as much or more at fault because he started the feud. The example is extreme, but Dupin nonetheless sends the quote to explain that although Dupin may have stolen the letter, the Minister was at fault because he committed the first crime.
Despite all the discussion concerning the whereabouts of the letter in "The Purloined Letter," the letter itself is merely a literary device around which Poe constructs a game of wits. The contents of the letter and its implications in the political sphere are not included because the plot does not need them, and any other object would have served just as well. Significantly, when Dupin finally finds the letter, the Minister has placed it carelessly into a rack hanging from the fireplace after folding it inside-out and making it appear insignificant. The manner of his hiding the letter is extremely relevant for the purposes of the story, but its inconsequential appearance reflects its relative importance in the novel. We might also consider it ironic that after all the fuss over the letter, its contents will never become any more public to the fictional world of Dupin than it will to the reader.