In medias res, or “into the middle of things” is Latin phrase used to describe a story that begins in the middle or even at the conclusion instead of at the beginning.
The main character and narrator of the story, Montresor, relates his story of revenge to an unnamed listener, the mysterious “you” who is acquainted with Montresor. He narrates his story in the past tense, referring to events that have already happened.
Finally, the clearest clue that Montresor is looking back and telling the story after the fact comes at the very end with these statements:
I forced the last stone into its position; I plastered it up. Against the new masonry I reerected the old rampart of bones. For the half of a century no mortal has disturbed them.
With the final sentence, we know that Montresor’s revenge happened fifty years ago--and no one has found and disturbed Forunato’s tomb since Montresor buried Fortunato alive in the wall.
I do not believe that this story really begins in medias res. Montresor is narrating it in the past tense. Everything happened fifty years before he tells about it. If Poe had wanted to tell the storyin medias res, he might have begun when Montresor and Fortunato were down in the catacombs with their torches. The story actually begins with a fairly conventional exposition in which Montresor mentions the thousand injuries of Fortunato and his own decision to exact revenge. After some considerable explanation of his thoughts about revenge, so characteristic of Poe, Montresor tells how he encountered Fortunato on the street during the carnival madness. From there on, the narrative is straightforward to the end except for a llittle coda in which Montresor tells how his crime has gone undetected for fifty years. I do not see how the term in medias res applies to this story.