Montresor wants revenge on one of his associates named Fortunato. “At length I would be avenged; this was a point definitely settled–but the very definitiveness with which it was resolved precluded the idea of risk.”
Montresor takes Fortunato down to the catacombs to share this rare Amontaillado he purchased.
They start to drink and have fun down there together. Montresor is getting him more drunk so he can lure him farther into the catacombs.
Montresor attacks fotunato and locks him inside and wall and patches it up, after he stabs him with his sword and set a flame just to make sure he was dead.
Montresor waited until the flame died just to make sure he was dead.
Fifty years later he tells the whole story as a tale and he gets away with the whole murder.