Montresor has one purpose and one purpose only: to secure revenge for the insult delivered to him by Fortunato. According to Montresor, Fortunato had heaped numerous "injuries" upon him, but his "insult" was not to be tolerated. The reader is never told what Fortunato's injuries and insults had been.
The opening paragraph of the story deals with the nature of revenge and Montresor's deep, overwhelming desire to be avenged for whatever it was that Fortunato had done to him. Montresor's revenge, to be effective, must be delivered by his own hand; Fortunato must realize that it is he (Montresor) who is exacting punishment. Montresor did not come up with this philosophy himself. It is part of his heritage. His family's coat of arms read Nemo me impune lacessit, meaning "No one wounds me with impunity." Revenge, it seems, was a family tradition.
Montresor's plan to avenge himself is diabolically clever. He lures Fortunato into the catacombs with the promise that he can taste an expecially fine wine in order to see if it is what it appears to be. He plays upon Fortunato's greed and his arrogance. Since Fortunato has been drinking already, he is easily managed. Once deep in the catacombs, Montresor chains him to a wall in a small chamber, then closes the opening to the enclosure with stone and mortar, laying one stone at a time with pleasure. As Fortunato comes to his senses, he realizes with horror what is happening to him and who is doing it. Montresor's revenge is complete.