The narrator introduces Dupin and talks about his personality and where they live together. He goes on to explain Dupins deduction skills and how he enjoys solving mysteries.
The bodies of Madame and Mademoiselle L'Espanaye are found. Madame was found in the courtyard and Mademoiselle was found stuffed up a chimney. Dupin becomes interested in the case because of the strange circumstances surrounding it and begins investigating.
After investigating the crime scene, Dupin and the narrator return to their shared apartment. Dupin begins to explain to the narrator that based on the inhuman strength that would have been needed to commit the crime he believes the culprit was not human. The finger prints on Mademoiselles neck leads Dupin to conclude that the murder is an Ourang-Outang most likely belonging to a Maltese sailor.
Dupin tracks down where the sailor is staying and accompanied by the narrator and French police, head to the building. Upon arriving, they enter without invitation. They hear a noise on the stairs confirming the sailor is inside. The sailor picks up a club as though he is going to fight the group.
The sailor eventually stands down to Dupin and agrees to tell explain how he was involved in the crime
The sailor explains how the Ourang-Outang Dupin believes to be behind the murders did indeed commit them. He explains how the ape escaped and murdered the two women. After clearing his name he gets the Ourang-Outang back.