In the short detective story The Purloined Letter, police know who stole a letter that contained valuable information, but are unable to find the letter itself. Some time later, the detective C. Auguste Dupin is able to hand over the letter to the police, having used logic and deductive reasoning to find it. While the police looked in real hiding places for the letter, Dupin decided to look in plain sight, often the best hiding place of all. The story is one of the earliest in the detective story genre.
The unnamed narrator is discussing with the famous Parisian amateur detective C. Auguste Dupin some of his most celebrated cases when they are joined by the Prefect of the Police, a man known as G—. The Prefect has a case he would like to discuss with Dupin.
A letter has been stolen from the boudoir of an unnamed woman by the unscrupulous Minister D—. It is said to contain compromising information. D— was in the room, saw the letter, and switched it for a letter of no importance. He has been blackmailing his victim.
The Prefect makes two deductions with which Dupin does not disagree:
The Prefect says that he and his police detectives have searched the Ministerial hotel where D— stays and have found nothing. They checked behind the wallpaper and under the carpets. His men have examined the tables and chairs with magnifying glasses and then probed the cushions with needles but have found no sign of interference; the letter is not hidden in these places. Dupin asks the Prefect if he knows what he is seeking and the Prefect reads off a minute description of the letter, which Dupin memorizes. The Prefect then bids them good day.
A month later, the Prefect returns, still bewildered in his search for the missing letter. He is motivated to continue his fruitless search by the promise of a large reward, recently doubled, upon the letter's safe return, and he will pay 50,000 francs to anyone who can help him. Dupin asks him to write that check now and he will give him the letter. The Prefect is astonished, but knows that Dupin is not joking. He writes the check and Dupin produces the letter. The Prefect determines that it is genuine and races off to deliver it to the victim.
Alone together, the narrator asks Dupin how he found the letter. Dupin explains the Paris police are competent within their limitations, but have underestimated with whom they are dealing. The Prefect mistakes the Minister D— for a fool, because he is a poet. For example, Dupin explains how an eight-year-old boy made a small fortune from his friends at a game called "Odds and Evens". The boy was able to determine the intelligence of his opponents and play upon that to interpret their next move. He explains that D—knew the police detectives would have assumed that the blackmailer would have concealed the letter in an elaborate hiding place, and thus hid it in plain sight.
Dupin says he had visited the minister at his hotel. Complaining of weak eyes he wore a pair of green spectacles, the true purpose of which was to disguise his eyes as he searched for the letter. In a cheap card rack hanging from a dirty ribbon, he saw a half-torn letter and recognized it as the letter of the story's title. Striking up a conversation with D— about a subject in which the minister is interested, Dupin examined the letter more closely. It did not resemble the letter the Prefect described so minutely; the writing was different and it was sealed not with the "ducal arms" of the S— family, but with D—'s monogram. Dupin noticed that the paper was chafed as if the stiff paper was first rolled one way and then another. Dupin concluded that D— wrote a new address on the reverse of the stolen one, re-folded it the opposite way and sealed it with his own seal.
Dupin left a snuff box behind as an excuse to return the next day. Striking up the same conversation they had begun the previous day, D— was startled by a gunshot in the street. While he went to investigate, Dupin switched D—'s letter for a duplicate.
Dupin explains that the gunshot distraction was arranged by him and that he left a duplicate letter to ensure his ability to leave the hotel without D— suspecting his actions. If he had tried to seize it openly, Dupin surmises D— might have had him killed. As a political supporter of the Queen and old enemy of the Minister, Dupin also hopes that D— will try to use the power he no longer has, to his political downfall, and at the end be presented with an insulting note that implies Dupin was the thief: Un dessein si funeste, S'il n'est digne d'Atrée, est digne de Thyeste ( If such a sinister design isn't worthy of Atreus, it is worthy of Thyestes ).