The Murders in the Rue Morgue, often considered to be the first true detective story, concerns two unsolved murders in Paris. No one can solve the mystery; the room in which the murders occurred was locked and supposedly impossible to get into. When a man is imprisoned, despite any evidence that he committed the crime, the protagonist Dupin offers his services. The story's themes include logical reasoning, deduction, and urban crime.
The story opens with a lengthy explanation of ratiocination. Dupin demonstrates his prowess by deducing his companion's thoughts as if through apparent supernatural power. The story then turns to the baffling double murder of Madame L'Espanaye and her daughter at their home in the Rue Morgue, a fictional street in Paris. According to newspaper accounts, the mother was found in a yard behind the house, with multiple broken bones and her throat so deeply cut that her head fell off when the body was moved. The daughter was found strangled to death and stuffed upside down into a chimney. The murders occurred in a fourth-floor room that was locked from the inside; on the floor were found a bloody straight razor, several bloody tufts of gray hair, and two bags of gold coins. Several witnesses reported hearing two voices at the time of the murder, one male and French, but disagreed on the language spoken by the other. The speech was unclear, and every witness admits that he does not know the language he claims to have heard.
Paris natives Dupin and his friend, the unnamed narrator of the story, read these newspaper accounts with interest. The two live in seclusion and allow no visitors. They have cut off contact with "former associates" and venture outside only at night. "We existed within ourselves alone", the narrator explains. When a bank clerk named Adolphe Le Bon is arrested even though no evidence exists pointing to his guilt (other than his delivering a goodly amount of money to the two ladies the day before), Dupin becomes intrigued and remembers a service that Le Bon once performed for him. He decides to offer his assistance to "G–", the prefect of police.
Because none of the witnesses can agree on the language the murderer spoke, Dupin concludes they were not hearing a human voice at all. He and the narrator examine the house thoroughly; the following day, Dupin dismisses the idea of both Le Bon's guilt and a robbery motive, citing the fact that the gold was not taken from the room. He also points out that the murderer would have had to have superhuman strength to force the daughter's body up the chimney. He formulates a method by which the murderer could have entered the room and killed both women, involving an agile climb up a lightning rod and a leap to a set of open window shutters. Showing an unusual tuft of hair he recovered from the scene, and demonstrating the impossibility of the daughter being strangled by a human hand, Dupin concludes that an "Ourang-Outang" (orangutan) killed the women. He has placed an advertisement in the local newspaper asking if anyone has lost such an animal, and a sailor soon arrives looking for it.
The sailor offers to pay a reward, but Dupin is interested only in learning the circumstances behind the two murders. The sailor explains that he captured the orangutan while in Borneo and brought it back to Paris, but had trouble keeping it under control. When he saw the orangutan attempting to shave its face with his straight razor, imitating his morning grooming, it fled into the streets and reached the Rue Morgue, where it climbed up and into the house. The orangutan seized the mother by the hair and was waving the razor, imitating a barber; when she screamed in fear, it flew into a rage, ripped her hair out, slashed her throat, and strangled the daughter. The sailor climbed up the lightning rod in an attempt to catch the animal, and the two voices heard by witnesses belonged to it and to him. Fearing punishment by its master, the orangutan threw the mother's body out the window and stuffed the daughter into the chimney before fleeing.
The sailor sells the orangutan, Le Bon is released from custody, and the prefect of police mentions that people should mind their own business once Dupin tells him the story. Dupin comments to the narrator that G– is " somewhat too cunning to be profound", but admires his ability " de nier ce qui est, et d'expliquer ce qui n'est pas " (a quote from Julie, or the New Heloise by Jean-Jacques Rousseau: "to deny that which is, and explore that which is not").