The story is about a man who takes revenge on his friend for insulting him by bricking him into the wall in the catacombs.

Montresor is angry at his friend Fortunato.  We have no idea why.  We do know that he intends to do something about it.

THE THOUSAND INJURIES of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge. You, who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat. At length I would be avenged….

Montresor tells Fortunato that he has a cask of rare wine, the Amontillado, that he wants his opinion on.  Fortunato follows him deep into the catacombs, or cavelike cemeteries underground.  Fortunato does not seem aware that he insulted Montresor, especially a thousand times.  If he suspected anything, he would not have gone into the catacombs with Montresor.

Once underground, Montresor tries to convince Fortunato that he is a Mason, but he doesn’t know the secret sign.  He also expresses concerns about Fortunato’s health.  Ultimately, Fortunato is so drunk by the time they reach Montresor’s destination that he does not really realize what is going on.  He thinks Montresor is playing a joke until he finally sobers up and realizes that Montresor is trying to kill him.  By then it is too late, and Montresor has walled him up.

The narrator is so caught up in the “perfect murder” that it never occurs to him that what he is doing is wrong.  In his mind, whatever Fortunato did to him justifies murder.  He is self-centered and homicidal, and he is a most unreliable narrator.  He is clearly insane.  That makes him creepy, but it sure makes the story interesting.