As the narrator, Montresor explains to an unknown audience that because Fortunato has mortally insulted him, he has vowed vengeance. However, he has hidden his animosity towards Fortunato because he wants to fulfill his vow without placing himself at risk, since the vengeance would not be complete if Fortunato were to retaliate or if Fortunato died without knowing Montresor to be his murderer. He maintains an appearance of good will towards Fortunato and decides to exploit Fortunato's weakness for fine wines.
One evening during the carnival season, Montresor finds Fortunato and invites him to try out a sample of sherry, which he has recently acquired and wishes to confirm as Amontillado. Fortunato is surprised and excited, so when Montresor suggests that Fortunato might be too busy and that Montresor might have Luchesi taste it instead, Fortunato insults Luchesi's skill with wines and insists on accompanying Montresor to the vaults to taste the Amontillado. Montresor offers a token protest, saying that the vaults are full of nitre and will aggravate Fortunato's cold, but the latter insists. Montresor puts on a mask of black silk and a cloak and leads Fortunato to his home.
Montresor previously told the servants that he would be gone all night and forbade them to leave, knowing that they would all disappear to join the carnival as soon as he left, so no one is home when they arrive. He takes two torches and, handing one to Fortunato, leads Fortunato into the Montresor catacombs. The passageway is damp and full of nitre, causing Fortunato to cough, but the dissembling Montresor shows false concern for Fortunato's health and offers him a sip of Medoc wine. Montresor offers to call for Luchesi, knowing that Fortunato will be insulted and insist on continuing. Fortunato drinks to the dead, and Montresor drinks to Fortunato's long life.
Fortunato notes the size of the vaults, and Montresor replies that the Montresors were a large family. They discuss the Montresor arms, which consist of a golden foot crushing a serpent that bites the foot against a field of blue; the Montresor motto is "Nemo me impune lacessit," the Latin phrase for "No one attacks me with impunity." As they walk deeper into the catacombs, Fortunato remains drunk and the bells on his costume jingle.
Montresor notes that the level of damp and of nitre is increasing since they are under the riverbed and offers to return to the surface, but Fortunato drinks more wine and throws away the bottle with a peculiar gesture. Montresor does not understand the gesture, causing Fortunato to realize that he is not a member of the Masons. Montresor insists that he is, although Fortunato is now incredulous, and he shows Fortunato the trowel that is in his cloak. Fortunato exclaims that Montresor jests but insists that they continue to the Amontillado. Montresor leads him to a deep crypt, at the end of which is a smaller crypt lined with human remains. In the fourth wall of the crypt is a small niche backed by walls of granite.
Fortunato tries to look into the recess, but because of the foulness of the air, the light from the torch is dim and he cannot see. Montresor claims the niche to be the location of the Amontillado and once again mentions Luchesi, which prompts Fortunato to insult Luchesi and walk drunkenly into the niche. Quickly, Montresor chains him to the granite while he is too surprised to resist. Montresor mocks Fortunato, asking him with false solicitousness about the nitre. Fortunato asks about the Amontillado, and Montresor agrees before he reveals a pile of building stone and mortar that has been hidden by bones. Montresor then begins to wall up the niche, with Fortunato inside.
Montresor lays the first tier of the wall before Fortunato revives with a moan from his drunken state. He then lays three more tiers before he hears another sound. Fortunato begins shaking his chain, during which time Montresor waits and listens, pleased. When Fortunato ceases clanking, Montresor lays three more layers of stone and uses his torch to look inside the niche. Fortunato screams, causing Montresor to hesitate, but after a moment he joins in the screaming until Fortunato stops. By midnight, only one stone remains, and before Montresor can complete the wall, Fortunato laughs weakly and asks Montresor to end the joke and free him, shouting, "For the love of God, Montresor!" but the latter mocks him, and Fortunato ceases to speak, despite Montresor's calls. The only thing Montresor can hear as he lays the last brick is the jingling of Fortunato's costume. Montresor's heart grows sick, which he blames on the damp catacombs, and he reconstructs the pile of bones, which no one disturbs for the next fifty years. Montresor ends his tale by wishing Fortunato a peaceful rest.
Upon a first reading of "The Cask of Amontillado," we might be tempted to view Montresor simply as an unreasonable, cold-blooded murderer. He presents us with only a vague understanding of his motivations, and his pretense of good will and careful manipulation of Fortunato indicates the care with which he has planned Fortunato's death. We again have a classic case of Poe's unreliable narrator, whose guilt and occasional irrationality prevents him from presenting himself truthfully to the reader. However, closer inspection shows that Montresor displays a particularly black sense of humor, with which he amuses both himself and the horrified reader as he leads Fortunato into his trap. He informs the audience of his intentions before he begins the story of his last encounter with Fortunato, and Poe employs both verbal and dramatic irony to convey the darkness of the story.
Verbal irony occurs when the literal meaning of what the speaker says contrasts heavily with the speaker's actual message. For example, Poe gives the victim the name of Fortunato, which may mean "fortunate" in Italian, but adds an extra element of cynical humor to Fortunato's jovial and unsuspecting character. Montresor's dialogue makes particular use of verbal irony, since he is aware that Fortunato has no idea what awaits him and thus will totally misinterpret Montresor's words. Montresor tells his victim, "My dear Fortunato, you are luckily met." Fortunato interprets these words to mean that Montresor is fond of him and is glad to run into him. Montresor, on the other hand, actually despises Fortunato and is only happy to see him because Montresor can now carry out his murderous plans. Furthermore, the word "luckily" also recalls the meaning of Fortunato's name and is thus entirely unfitting for Fortunato's fate. Other examples of verbal irony include Montresor's showing of the trowel to Fortunato to prove he is a Mason; Montresor is about to become a mason by imprisoning Fortunato, but he is not a Freemason.
Because both the audience and Montresor are aware of the unfortunate Fortunato's impending death, dramatic irony also plays a role in the comedy of horrors of "The Cask of Amontillado." Dramatic irony is the result of the disconnect that occurs when a character, namely Fortunato, is not aware of the true meaning of his own actions. The very setting of the story is ironic, in that Montresor has chosen the jovial carnival season to enact his murder because no one will be at his estate to witness the crime. Fortunato himself is dressed in a jester's outfit, and the jingling of his jester's bells remind us of the atmosphere of happiness and cheer outside the catacombs. Later, as they drink the Medoc, Fortunato drinks to the dead and buried, not realizing that he is about to join them, and Montresor wryly drinks to Fortunato's health.
The key to the humor in "The Cask of Amontillado" is that despite Montresor's sardonic jabs, Fortunato does not realize the extent of his danger until he has been chained to the granite, and even then he remains too drunk to completely comprehend what has taken place for some time. After repeatedly insulting Luchesi for his lack of intellect, Fortunato shows himself to be even more the dupable fool. Because of Fortunato's drunken and therefore unsuspicious condition, we do not know if Fortunato would have been any cleverer in his normal state. Nevertheless, by the end of the story, Montresor shows himself to be both the more villainous and the more intelligent being. As he tells Fortunato, he comes from a family with a motto and a coat of arms that indicates a long tradition of revenge, and he ignores any pangs of heart sickness by blaming the damp and shutting Fortunato into the burial ground of his avenging family.
As in many of Poe's short stories, Montresor is the first-person narrator and appears to be speaking to a specific audience. However, whereas we can suppose that the narrator of "The Tell-Tale Heart" is speaking to some authority figure in order to prove his sanity, in "The Cask of Amontillado" we know very little about Montresor's audience or motivations. The only hint we have comes in the first paragraph, where he implies that his audience already knows something of Montresor's thoughts and personality. The account occurs some fifty years after the event, suggesting that a somewhat older Montresor was never discovered and has not greatly changed his opinion that the crime was justified. Montresor has shown himself to be risk averse, so his audience must be someone that he trusts, perhaps a confessor or a relative. Possibly he is at the end of his life, and now that he can no longer face any severe consequences, he has decided to tell his story. The ambiguity of the circumstances and Montresor's escaping of justice lend a sinister tone to his story, which is further backed by Poe's extensive use of irony.