Montresor tells the story of the night that he took his revenge on Fortunato, a fellow nobleman. Angry over some unspecified insult, he plots to murder his friend during Carnival when the man is drunk, dizzy, and wearing a jester's motley.

He baits Fortunato by telling him he has obtained what he believes to be a pipe (about 130 gallons) of Amontillado, a rare and valuable sherry wine. He claims he wants his friend's expert opinion on the subject. Fortunato goes with Montresor to the wine cellars of the latter's palazzo, where they wander in the catacombs. Montresor offers De Grave, a wine, to Fortunato; at one point, Fortunato makes an elaborate and—to the narrator's eyes—grotesque gesture with an upraised wine bottle. When Montresor appears not to recognize the gesture, Fortunato asks, "You are not of the masons?" Montresor says he is, and when Fortunato, disbelieving, requests a sign, Montresor displays a trowel he had been hiding.

Montresor warns Fortunato, who has a bad cough, of the damp, and suggests they go back; Fortunato insists on continuing, claiming that "[he] shall not die of a cough." During their walk, Montresor mentions his family coat of arms: a foot in a blue background crushing a snake whose fangs are embedded in the foot's heel, with the motto Nemo me impune lacessit (No one attacks me with impunity). When they come to a niche, Montresor tells his victim that the Amontillado is within. Fortunato enters and, drunk and unsuspecting, does not resist as Montresor quickly chains him to the wall. Montresor then declares that, since Fortunato won't go back, he must "positively leave [him]."

Montresor walls up the niche, entombing his friend alive. At first, Fortunato, who recovers from his drunken state faster than Montresor anticipated he would, shakes the chains, trying to escape. The narrator stops working for a while so he can enjoy the sound. Fortunato then screams for help, but Montresor mocks his cries, knowing nobody can hear them. Fortunato laughs weakly and tries to pretend that he is the subject of a joke and that people will be waiting for him (including the Lady Fortunato). As the murderer finishes the topmost row of stones, Fortunato wails "For the love of God, Montresor!" Montresor replies, "Yes, for the love of God!" He listens for a reply but hears only the jester's bells ringing. Before placing the last stone, he drops a burning torch through the gap. He claims that he feels sick at heart, but dismisses this reaction as an effect of the dampness of the catacombs.

In the last few sentences, Montresor reveals that it has been 50 years since the murder, he has never been caught, and Fortunato's body still hangs from its chains in the niche where he left it. The murderer, seemingly unrepentant, ends the story by remarking: In pace requiescat (may he rest in peace).