Poe's story has a (not so) hidden riddle.
A (not so) romantic one, presented chastely.
That is, the story provides a double puzzle. There is the puzzle which is solved by Dupin, and this story is told by the narrator completely. But there is another detective story not told completely, because it is a puzzle the reader is supposed to solve.
It begins with
the nature and content of the letter.
What kind of letter is it? Why doesn't the narration tell us that basic information? Isn't it important? It is. So? -- Certainly the author wants us to figure it out. Is it possible? I think it is.
Since the whole story is about the letter, there is no reader who wouldn't ask the question what the letter is about. There is no writer unaware of this. This "forgetfulness" of the author is clearly a provocation. He calls the reader to investigate, to be a detective like the hero. Poe's work is not just theory, it's also practice. While it solves a puzzle for you, it creates another one to be solved by you.
Four facts we learn concerning the letter, which may indicate its possible content:
(1) it is written to and received by the Queen (never called so in the text, but it's clear)
(2) the sender's seal "was small and red, with the ducal arms of the S family".
(3) The Queen tries to conceal the letter from the King and Minister D (and probably anyone), and, when stolen by Minister D, she wants to get it back badly, and in full secrecy.
(4) Minister D (secretly) blackmails the Queen with the letter.
From the fact (3) that the Queen's trying to conceal it, we have basically two possible/likely assumptions/solutions:
(A) the letter is part of a political conspiracy, or
(B) it is from her lover.
If someone finds further likely assumptions, they should go with it.
Very few things, maybe nothing, in the narrative hints at (A). Though Prefect G says that the thief, Minister D, tries to use the letter for dangerous "political purposes", this vague description is just what an intelligence agent must say if he wants to conceal the fact that it's a love letter, a proof that the Queen has an adulterous liaison. Except for this statement, not to be taken at face value, hardly can we find any sentence in the whole text indicating that the Queen would be involved in any political conspiracy against the King. The narrative does not treat political issues, views, motifs -- as it does treat logical, methodological, ethical, aesthetical, epistemological, psychological, and, more covertly, romantic issues, views, motifs.
I think it the far most realistic assumption that the purloined letter is a love letter. So the whole issue is, more or less, a love issue.
Love! Making the reader immediately bump into a legislative (etc.) question: isn't it committing treason (by the Queen, Minister D, Prefect G, and Dupin) to conceal the letter from the King? Or love would be that precious thing evading even legislation?
(In classic romances we find the same king-queen-lover triangle: Marke-Isolde-Tristan or Arthur-Guinevra-Lancelot. Or the troubadour's Lady is also usually married, though this poetry is not about having sex with her, let alone advocating adultery. However, adultery is one of the four marks of courtly love according to C.S. Lewis: Humility, Courtesy, Adultery, and Religion of Love. The hidden story of "The Purloined Letter" is clearly an example of the classic romance.)
As for the content or nature of the letter, some interpretations of Poe's story have come to the same conclusion. It is curious, however, that you can hardly find an interpretation that would treat the similarly evident question about
3.
the content or nature of the blackmail.
A more covert and obvious riddle is what this "monstrum horrendum", Minister D, may have asked in exchange for not revealing the purloined letter. What is it he is trying to blackmail her into?
The same narratopoetic question arises as in the case of the letter. The whole story is about this letter -- and we don't get the basic information of its content or nature? The whole story is about Minister D's blackmailing the Queen -- and we don't get the basic information of its content or nature? Well, the narration seems to be interactive. It seems to provoke the reader's activity, detective work to figure the answers out.
This puzzle is far less simple to solve than that of the nature of the letter . However, if we read the signs carefully, sensitively, intelligently, like Dupin (or his Creator), we can do that. Many little things "of no importance" hint at the quite obvious solution.
To find out what can be the content or nature of Minister D's blackmail, let's see some telling features we learn about Minister D.
1. He is a well-informed intriguer who knows about the Queen's secret liaison from the very start of the story: "His lynx eye immediately perceives the paper, recognises the handwriting of the address, observes the confusion of the personage addressed, and fathoms her secret." He is aware of that intimate issue even her husband, the King is not.
2. Minister D "dares all things, those unbecoming as well as those becoming a man" -- as Prefect G puts it. This well-informed insider holds Minister D a manly person, but far from always being a gentlemanly one. As opposed to G's humbug about D's "political purposes", G's words cited above may reveal more about the real nature of the issue, and possible motifs of D.
(Excursus: Prefect G, in his petty, infantile, unmanly way, is in sort of love with the Queen. He is her most ardent servant, as we can discern it from his every word about her. He is all too eager to attract her, as we see when he gets the letter from Dupin and immediately and zealously runs to the Queen with it, forgetting even to ask Dupin how the hell he acquired it. Isn't Prefect G curious about it? Isn't it of high importance for him, a person so proud of his detective professionalism? Of course, he is curious, it is of high importance for him. That's why his zealous forgetfulness is so telling. -- By the way, Dupin also calls himself "a partisan of the lady concerned", so the Queen must be an enchanting personality.)
3. What can we deduce from "the habits of the minister" (which give Prefect G and his men "a great advantage", letting them search for the letter undisturbed in D's apartment), that "he is often absent from home all night"? What can D do out there at night, all night? There can be other answers, but the most likely guess, I think, is that he is with a woman; he is regularly out there for his lover(s). This fits perfectly in the character of D, and in the untold romantic theme the story arises.
4. Minister D not only keeps the purloined letter outside the assumable hide/search domain, but also applies some disguise/camouflage on it: he changes the seal's color, he makes it a little dirty and torn, and on the outside it "was addressed, in a diminutive female hand, to D, the minister himself." So he is a man having a female acquaintance who can be enticed to write on the back of the Queen's love letter, to help Minister D to disguise it and to blackmail the Queen. (This makes us associate it with the world of [the sex-oriented, but all-encompassing amorality of] Vicomte de Valmont and Marquise de Merteuil in Dangerous Liaisons.)
5. Minister D is "an unprincipled man of genius", as Dupin puts it. He has no principled political goals either. Politically, socially, he is at the top he can and wants to be. (Probably the Minister of Culture, though not mentioned.) Above him there is only one level, the royal pair. No way he wants to be the king. The king can not afford, as he can, to go out every night, or to tinker with poetry and mathematics, to play the instruments (that Dupin saw on his table) etc. Minister D is more of a bonvivant type. For such a man, a bold villain, probably a womanizer, there can hardly be any other way to step upward, one possible ambition, one way to obtain more power a real challenge: to have the Queen.
There are further loci in the text that (only) allows this line of interpretation and conclusion: the objective of D's blackmailing the Queen is of adult nature. The ransom for not revealing the purloined letter to the King (or to the public) is that she would get laid by D: he wants her (to have sex with him).
4.
But the Queen remains a true lover. Faithful to her monogamous love / adultery. And chastity conspires with that.
The hidden story in "The Purloined Letter" follows the rules and traditions of courtly love, of classic romance and chivalric poetry, which were the first to emphasize the power of love in the context of marriage (which had nothing to do with love before modernity) and Christianity (which, otherwise, was the most stringent in requiring marital fidelity). In this new religion of love, the beloved being is the absolute value, the exclusive motive which cannot be overcome by any other, however strong, values or motives -- even loyalty to the king or the spouse, not to mention sheer lust, power, violence, money or blackmail.
Note
Jacques Lacan and Daniel Hoffman are two readers of the short story believing (without a detailed analysis) that it is a love letter. Lacan remarks that the letter is "one of those correspondences whose secrecy is sometimes necessary to conjugal peace" ("Seminar on 'The Purloined Letter,'" transl by Jeffrey Mehlman, in The Purloined Poe: Lacan, Derrida, and Psychoanalytic Reading, ed John P Muller and William J Richardson, Baltimore: Johns Hopkins Univ Press, 1988, p 33.) Hoffman says that "we are never told the contents of the purloined letter. What, however, could it be but a love-letter received by her [the Queen] from a suitor not her husband?" (Poe, Poe, Poe, Poe, Poe, Poe, Poe. Baton Rouge: LSU Press, 1972, p 131.)
