Montressor wants to get revenge on Fortunado because he insulted him and the way he is going to get back at him is by killing him. Montressor knows that Fortunado's weakness is wine so he tells Fortunado that he bought a bottle of what seems to be an amontillado.
Montressor tells Fortunado if he can take a look at it so see if it's an amontillado. Fortunado agrees and they go down to the catacombs.
Montressor keeps asking Fortunado if he wants to go back because his cough is getting really bad but Fortunado insists to keep going.
Montressor chains Fortunado and he starts sobering up because he starts realizing what is happening.