For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Montresor thinks Fortunato insulted him, and he vows to wreak a terrible vengeance on the man.
Act II
He lures Fortunato down underground, into the foul catacombs of the Montresor family, with the promise of Amontillado. Then he gets Fortunato really drunk.
Act III
After a long and drunken journey with lots of human remains, including piles of bones, Fortunato walks into a hole in the wall. Montresor proceeds to chain him up and brick him in. After that we learn that the whole thing actually happened fifty years ago…