What's the plot of the story "The Cask of Amontillado" by Edgar Allan Poe?

The author, posing as a man named Montresor, tells the reader all about his motive in the first sentence of the story.

THE THOUSAND INJURIES of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge. 

The plot is solely concerned with how Montresor will achieve his revenge. He adds in the opening paragraph:

At length I would be avenged; this was a point definitely settled...

Poe wanted that point "definitely settled" so that he could focus on Montresor's logistical problem, which was to lure Fortunato to his death in the catacombs without ever being suspected of the crime. The story is entirely about how Montresor accomplishes his goal of killing Fortunato without being caught, or even suspected. It involves a totally fictitious cask of Amontillado. Montresor encounters his victim on the streets, lures him to his palazzo with the gourmet wine he supposedly has stored underground, keeps him drunk and distracted, manipulates him with reverse psychology, leads him to a niche where he chains him to the rock wall and leaves him to die. The story ends with the narrator, Montresor, bragging about his complete success in committing the crime.

Against the new masonry I re-erected the old rampart of bones. For the half of a century no mortal has disturbed them. In pace requiescat!

The whole plot has to do with the means by which Montresor commits his perfect crime of murder. The fact that no mortal has "disturbed" the rampart of bones in fifty years is proof that Montresor was totally successful in accomplishing what he intended to accomplish. In the opening paragraph of the story he explains that intention.

I must not only punish but punish with impunity. A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong.

Although Montresor may be a sadistic monster, the reader identifies with him because he is held in Montresor's point of view from beginning to end. Point of view is the best way of attaining reader identification. Further, the reader knows Montresor's grievances and his motivation. The reader is put in the position of being the only person in the whole wide world who knows about Montresor's guilt and who knows the location of the body--except for Montresor himself, if he is still alive. In effect, the reader is an accomplice.