Booker says that this stage of the Quest plot begins in a "dark city," plagued by "corruption." Hmm. Let's see. We have (1) a royal lady, (2) paying the head of the police to (3) steal a letter that was (4) stolen from her by a (5) government official. Said government official is using the letter to (6) pressure the royal lady into making his political desires reality. Sounds like corruption to us.
Booker also says that the hero of the story (we'll use Dupin) can "only rectify matters by making a long, difficult journey." He might get "supernatural or visionary direction" to help prepare him. So, yes, Dupin is called on, rather indirectly, to help find the stolen letter. "Supernatural or visionary direction"? Well, his discovery does seem rather remarkable.
But we're pretty sure this isn't going to "rectify" much of anything.
How much of a journey can this story really take, when most of it happens in the library? Well, it's a journey of the mind. As Dupin contemplates D—'s psychology, he journeys into the consciousness of another person.
And here's something kind of cool: Booker says that, after The Call, the hero sets out on his journey, probably across "hostile terrain." He faces "life threatening ordeals." Doesn't sound much like "The Purloined Letter," right? Well, in order to figure out the mystery, Dupin actually has to try to become as much like D— as possible. In the words of the eight-year-old boy he quotes in his anecdote to the narrator, he has to "fashion the expression of [his] face"—or in this case, his mind—"as accurately as possible, in accordance with the expression of [D—'s]."
In other words, Dupin has to think, act, and look as much like D— as possible. And check out what he does: casually purloin a letter, right in plain sight, and then use it to extort money from someone else. This is beginning to sounds like a pretty dangerous game to play.
In this stage, the hero has the main goal in sight but has to overcome "terrible series of obstacles" before he reaches it. That sounds about right: Dupin sees the letter, but he can't just take it—he has to be sneaky. He has to figure out a way to purloin it.
Of course, Dupin doesn't seem particularly stumped by these obstacles. Au contraire, he enjoys the game. Not only does he get to do a fun craft project with the fake letter, but he gets to stage a mock street shooting to distract D—. It's almost too easy.
Quest heroes must get awfully tired of all their ordeals. In this case, Dupin's test is getting the letter without alerting D—. Check and check. Final ordeal, passed.
By the end, Dupin's scored a triple hitter: fifty-thousand of G—'s francs, the gratitude (presumably) of the royal lady, and a thumbed nose at his (apparently) mortal enemy, Minister D—. Not too shabby for a hero who only had to leave his armchair twice.