Montresor thinks Fortunato insulted him, and he vows to wreak a terrible vengeance on the man.
He lures Fortunato down underground, into the foul catacombs of the Montresor family, with the promise of Amontillado. Then he gets Fortunato really drunk.
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…