The narrator meets Dupin in a small library in Montmartre, looking for the same book.
The two become roommates, living together in a mansion in the Faubourg St. Germain.
While they live together, the narrator and Dupin spend all of their daylight hours with the blinds drawn reading and talking to each other. At night, they prowl the streets of Paris staring at people.
During one of these outings, the narrator expresses amazement: he and Dupin had been walking in silence for several minutes when Dupin suddenly identifies what the narrator's thinking, out of the blue. The narrator uses this fact as proof of Dupin's creative reasoning abilities.
The narrator is fascinated by a newspaper article of a series of "Extraordinary Murders" in a house on the Rue Morgue. The two victims are a mother and daughter, Madame and Mademoiselle L'Espanaye.
The narrator collects all the details of the case from the paper. We learn that series of witnesses testify to a series of horrible screams at around 3am that stop when the neighbors break into the door.
The witnesses then hear two voices arguing, one of a Frenchman and one of an unrecognizable "foreign" origin.
Once the neighbors break through the door of the fourth story apartment, they find Mademoiselle L'Espanaye stuffed up a chimney, Madame L'Espanaye's body mutilated and nearly decapitated in the rear courtyard, and four thousand francs in gold. All the windows and doors are locked from the inside.
The narrator is confused by these facts, but he agrees to accompany Dupin on a search of the apartment anyway.
There, the narrator sees nothing that the newspapers didn't report, but he does observe that Dupin has examined everything with great care.
The narrator is even more amazed when, the next day, Dupin announces that he expects a visitor soon who will explain these crazy murders.
Meanwhile, Dupin explains his own thought processes, which lead him to convince the narrator that the killer was an Ourang-Outang.
The narrator waits in astonishment for a man who Dupin says will be a sailor.
Sure enough, the sailor appears, and the narrator jots down his account of the events that lead the sailor's ape to kill the L'Espanayes.
After this, all has been solved: the narrator closes the story with Le Bon (the original suspect) getting off the hook and Dupin feeling smug at the expense of the police.