Montresor approaches Fortunado at the carnival and he tells him that he has a bottle of Amontillado. Fortunado's weakness is alcohol/wine and he is very good at identifying it. Montresor asks Fortunado if he would like to come and see if he got a good deal on the Amontillado. Fortunado is quite drunk and agrees to go even when Montresor tells him that he can go and get someone else.
Montresor's plan was to get revenge on Fortunado for insulting him. He planned on getting revenge by killing him down in his catacombs while nobody was home.
As they are going down into the catacombs, Fortunado starts getting a cough because of the mold and Montressor uses reverse psychology by inisting that they go back up. Fortunado insists that they continue. As they continue, Fortunado's cough gets worse and Montressor gives him some wine to ease his cough. Fortunado insults Montresor again as they continue down.
When they reach the end of the catacombs Montressor puts Fortunado in an opening in the wall and chains him up. Fortunado is still drunk so he doesn't really process what is going on. Montresor starts to wall him in and Fortunado starts to sober up.
Fortunado sobers up and is crying an pleading for help so Montressor mocks him. eventually, Fortunado goes silent and Montresor continues. Fortunado tries one last attempt to get free by telling Montresor that it was a good joke and that they should get back to the carnival. Fortunado falls silent so Montresor finishes up the wall and exits the catacombs.
After 50 years, nobody has accused Montressor or even been in the catacombs. Montressor's plan worked out and he got his revenge.