Montresor, the narrator, says that he has decided to get revenge against Fortunato because he has suffered a "thousand injuries" and some insults from him which he does not describe explicitly. Some critics have suggested that Montresor is insane and that the injuries and insults are purely imaginary. This is a moot question.
Fortunato's weakness, according to Montresor, is his egotism, especially with regard to his connoisseurship in fine wines. Montresor uses Fortunato's expertise in wines to lure him into the catacombs beneath his palazzo, where he claims to have stored a big cask, a "pipe" containing 126 gallons, of what he thinks to be rare imported Amontillado sherry. He is asking Fortunato to taste the wine and give his opinion because, as he repeats, "I have my doubts." Fortunato wants to drink some of the sherry for pleasure, also to prove his supreme connoisseurship, and probably most importantly to buy some for himself and for resale if it indeed proves to be the true Amontillado.
Fortunato is drunk when Montresor finds him in the street during a big carnival and keeps him drunk by giving him more wine when they are underground. He leads Fortunato to a narrow niche where there are two short chains attached to the rock wall. Evidently they have been there for many years and have been used on other victims. He guides the befuddled Fortunato into the niche and padlocks the two chains around his waist, pinning him against the rock wall. He then proceeds to build a stone wall to close off the niche from ground to ceiling with mortar and stones he has prepared in advance. He leaves his victim to die of starvation, and he is writiing about his successful revenge fifty years after the event.