The narrator opens with a short discussion of the analytical mind, whose conclusions seem to be the result of intuition. He compares the task of analysis to a game of draughts (checkers) rather than one of chess, arguing that a good chess player benefits from concentration whereas a draughts player benefits from intelligence; because the number of possible moves are limited in draughts, a player will win by analyzing every possibility and by observing his opponent. The analyst, claims the narrator, is one who can maneuver his opponent into error by identifying with his opponent and by viewing all possibilities. Furthermore, ingenuity does not always include analysis, although analysis is always ingenious. These two qualities are different in that the ingenious are fanciful whereas the analytical are imaginative.
To explain his point, the narrator offers an account from his acquaintance with a Monsieur C. Auguste Dupin, who is living parsimoniously in Paris. Nevertheless, Dupin often splurges on books, and he meets the narrator in the Rue Montmartre because both of them are searching for a rare volume. They quickly become friends and decide to live together in seclusion in a gloomy old mansion, for which the wealthy narrator pays the rent. The narrator notices Dupin's superb analytic ability, which Dupin attributes to his understanding of people's thoughts. On one of their customary nighttime walks, Dupin displays his skill by responding to a comment that the narrator had not asked aloud, about a former cobbler who had entered the stage to much ridicule. Surprised, the narrator asks him how he came to the correct conclusion, and Dupin explains that when the narrator ran into a fruiterer, he commenced a series of thoughts that Dupin had deduced by reading his friend's body language and by recalling former conversations.
Soon after Dupin displays his mental prowess, they observe a news article about a set of murders which had occurred that morning at three A.M. Neighbors of a house in the Rue Morgue heard screams from the fourth story of a house belonging to Madame L'Espanaye and her daughter Camille. After several attempts at entering the house, eight to ten of the neighbors and two gendarmes succeeded in opening the gateway with a crowbar, and although the shrieks had suddenly ceased, they entered to hear multiple angry voices from the upper portion of the house that soon faded away. They searched the house to find a room on the fourth floor that was locked from the inside. The room was totally destroyed and contained, among other things, a bloody razor, clumps of grey hair, and two bags with four thousand francs in gold. The bureau's drawers were open and damaged, and an iron safe with a few minor papers was found open under the mattress, which had been thrown on the floor. The daughter's fresh corpse had been forced feet first into the chimney; her face was scratched, and her throat was bruised as if she had been strangled. The mother was found in the backyard with her throat so deeply cut, probably by the razor, that her head fell off when she was picked up, and with her body extremely mutilated by some heavy, blunt object.
The next day, the papers offer the testimony of various witnesses. A laundress and a tobacconist testify that the old lady and daughter were fond of each other, that they seemed to have money, and that they did not usually entertain visitors and had no regular servants or any furniture except on the fourth floor. The L'Espanayes had no living relatives and had lived there for over six years. A gendarme claims that one of the voices was gruff, male, and French, and the other shrill and strange. No one can confirm the shrill voice's accent, which they all agree is foreign, but they claim that the French voice shouted, "sacré" (holy), "diable" (devil), and "mon dieu!" (My God!). A banker reveals that Madame L'Espanaye withdrew four thousand francs in gold three days prior to her death. All agree that the fourth floor chamber was locked from the inside and that all potential points of entry were closed and fastened. No people were heard or observed in the few minutes after they heard the voices and before they forced open the door to the chamber. Adolphe Le Bon, the clerk who brought Madame L'Espanaye the four thousand francs, is arrested despite a lack of evidence.
Dupin is very interested in the case because of its seeming impossibilities and because Le Bon once did him a favor. He criticizes the Parisian police for being cunning but too involved in the details of the case to see clearly. He knows the Prefect of Police and obtains permission to investigate for himself. He and the narrator visit the Rue Morgue. Dupin carefully examines all the evidence but says nothing until the next day, when he asks his friend if he saw anything peculiar and again criticizes the police for being too confused by the lack of an obvious motive and the murder's atrocity to use these oddities to look for an unusual solution. He reveals his pistols, telling the narrator that he is awaiting the arrival of someone whom he suspects will be closely connected to the crime. Speaking in an abstract tone, he points out that the voices were not female and therefore it could not have been a murder-suicide before noting that although the witnesses were of different nationalities, all of them identified the shrill voice as a foreign one. None of them recognized any distinguishable words.
Dupin then notes that the police have failed to observe that the murderers must have disappeared through the windows in the chamber. Although they appear to be locked by nails, Dupin concludes that they must have a concealed spring that allows the windows to fasten themselves. One of the two windows is half-hidden by the bedstead, and investigation reveals the nail to be broken, even though it appears whole when the window is down. The next matter, Dupin explains, is to determine how the intruders could have entered from the window. The police believe that no one could have climbed up the wall, but Dupin sees that someone of extraordinary athletic ability could have climbed up a nearby lightning rod and jumped onto a window shutter.
Continuing with his explanation, Dupin suggests that the motive of money is unlikely, since no one took the money, and the bureau's drawers might not actually be missing any articles. As Dupin tells the narrator, the case has a culprit with an unidentifiable voice, superhuman agility, a penchant for butchery, and no significant motive. No human would have been able to stuff the daughter so firmly up the chimney, pull such great clumps of hair from the old lady's head, or slit the lady's throat with so much force from a razor. The force that mangled the old woman's body must have been the collision with the ground, since she must have fallen out the window, a fact that the police never considered due to their belief that the window was sealed. The narrator suggests that the culprit might have been a madman, but Dupin shows him a tuft of hair from the fingers of Madame L'Espanaye that is clearly not that of a human.
Dupin then reveals a sketch he made of the bruises and fingernail marks on the victim's throat, which indicate the presence of a hand that is too large for a man and exactly matches the paw of an Ourang-Outang. An Ourang-Outang, according to the narrator, would have the brutality and the voice to fit the identity of one of the intruders. The other man, who was heard to shout "mon dieu," must have been horrified and probably tried unsuccessfully to recapture the animal. Dupin suspects that the Frenchman is probably innocent, and has put out an advertisement claiming that he has captured an Ourang-Outang and wishes to return it to its owner. Next to the lightning rod at the Rue Morgue, Dupin finds a ribbon featuring a sailor's knot that is common among the Maltese, so he deduces that the man is probably a sailor from a Maltese ship. As Dupin predicts, a French sailor arrives to pick up the animal, but Dupin locks the door and takes out his pistol, commanding the sailor to tell the truth about the deaths at the Rue Morgue. Dupin tells the sailor that he believes the sailor is innocent but wants him to confess his knowledge so that Le Bon will not be falsely charged with murder.
The sailor admits to having captured the Ourang-Outang in Borneo and brought it to Paris, keeping it in his apartment until he could sell it. One night, the animal broke free and was holding a razor in an attempt to imitate shaving, which it had previously observed its master doing. The sailor tried to calm the animal with a whip, but his actions provoked it into escaping. He chased it to the Rue Morgue, where it entered the house as Dupin had surmised, and he climbed up the rod, where he witnessed the murders. The women had been reorganizing the papers in the iron chest when the Ourang-Outang seized the older woman by the hair and waved the razor around, while the daughter fainted. The mother's screams enraged the animal, and it slit her throat and strangled the girl. It then saw the sailor watching through the window and became fearful, so it rampaged nervously, damaging the room and dragging the mattress from the bed. It then shoved the girl's corpse up the chimney and threw the mother out the window. After shouting in horror, the sailor ran away. The animal escaped out the window before being seen, closing the window behind it.
The owner later recaptures the Ourang-Outang and sells it to a zoo. Le Bon is released, although the Prefect of Police is embarrassed and resentful and consequently is somewhat sarcastic about Dupin not minding his own business. Dupin ignores the jab and reiterates his belief that the Prefect has too much cunning and ingenuity and not enough analytic ability. He quotes Rousseau, saying that the Prefect has a way "de nier ce qui est, et d'expliquer ce qui n'est pas" (denying what is and explaining what is not).
With "The Murders in the Rue Morgue," Edgar Allan Poe introduces the prototype of the quintessential detective, in the form of Monsieur C. Auguste Dupin, whose theory and powers of analysis are displayed by his skill in solving a seemingly intractable case and explained by his friend's opening monologue. Poe refers to Dupin's method as ratiocination, in which Dupin uses not only logic but also creativity in solving his case. As with Sherlock Holmes and Hercules Poirot, two fictional detectives that later follow Dupin's lead, Dupin allows the police to do most of the grunt work before stepping in for his own investigations and formulating his theories from his home rather than from a police station. He disparages the police for lacking creative insight because the key to Dupin's analytical aptitude lies in his ability to imagine the mind of his opponents and to use his understanding of how others think to reconstruct their thoughts - and therefore their actions - in his mind.
Dupin's use of creativity in solving the case reflects Poe's background and strengths. Although Poe is very fond of creating and solving puzzles, he is essentially a writer, editor, and critic with an aesthetic sensibility and an interest in exploring the psyches of murderers and madmen. Thus, Dupin acts to some extent as Poe's doppelganger, the character who strives to understand the mindsets of others in order to construct a story that logically follows from their character and from the given circumstances. The narrator is a man who appears to be of above-average intelligence, but he lacks the spark of creativity that would have been prized by a writer whose livelihood depended on his creative output, and as such, he is a foil for Dupin's brilliance. Dupin cannot serve as the narrator because to hear his unfinished thought processes would detract from the final reveal of his perfectly devised solution.
When the narrator discusses Dupin's mental talents, he briefly mentions the idea of "the old philosophy of the Bi-Part Soul," and speculates that Dupin has two sides to his soul, "the creative and the resolvent." As is common in Poe's stories, we have the idea of a doubled self, and the narrator gives a visual description of Dupin in his resolvent stage, in which he appears "frigid and abstract" and gives a sense of being emotionally removed from his audience. When he explains his solution to the narrator, he gives the impression of being an entirely different person. At the same time, this duality in Dupin's character echoes his method of attempting to understand people and determine their train of thought. When Dupin uses his creative side to identify with others, he allows himself to become a different person, generating new dualities in our understanding of his personality.
Dupin looks down on the policemen for their lack of analytical skills and because they allow themselves to be too distracted by the horrific nature of the crime to consider alternative possibilities for the source of the crime. By contrast, Dupin understands how to separate his emotions from his logic. Interestingly, this distinction is complicated by the fact that the culprit turns out to be an orangutan, a higher-level primate that represents a branch of evolution from an earlier path. Because the orangutan is a wild beast with little to no control over its emotions, we can interpret its existence in two ways. First, we can posit that whereas the orangutan serves as an example for the passions of the animal, Dupin is a specimen of further evolution, using his brain to attain a level beyond that of his friends and acquaintances. On the other hand, Dupin uses his clinical approach to look beyond the ordinary only to find that pure animal emotion is the solution to his method of emotional removal, and this juxtaposition indicates a powerful irony.
In general, Dupin is the opposite side of the coin from the insane criminals of Poe's other stories. Whereas Poe's psychotic protagonists cannot even comprehend the logic of their own thoughts and actions, Dupin specializes in understanding exactly these areas in the minds of others. In addition, although the narrator of "The Black Cat" is similar to Dupin in that they both seek plausible solutions to what appear to be inexplicable or otherwise supernatural situations, Dupin is working to discover the truth even though his murderous counterpart wishes to hide it. Fittingly, Poe chooses to give the guilty madmen the advantage of the first-person point of view and withholds this advantage from Dupin, but in the end, we believe Dupin far more readily than we believe the others.