We learn about the narrator and Dupin. They live together in Paris and only go out at night. We learn about the murders and each character/suspect is explained in depth. There are twelve characters introduced.
Adolphe Le Bon is arrested by police because he delivered francs to Madame L'Espanaye three days earlier and is thought to be the culprit. Dupin feels like he owes something to this case and that he must solve it.
The narrator and Dupin go to the apartment to investigate the murders and look for more clues. The narrator is baffled at how the women could've been killed and Dupin is also far from solving the case.
Dupin gets a man with evidence of the murder to come to the apartment to reinforce his proof that he already has. Dupin has solved the case but doesn't say yet who did it. We find out that the man who had evidence was a sailor that owned and orangutan that murdered the women.
The sailor brought the orangutan to Paris to sell it but it had escaped from his house and as he was chasing it around the city it saw the light in the window and swung in and killed both women. He cut Madame L.'s throat with a razor blade and strangled Mademoiselle L. The sailor never came forward because he didn't want to be blamed for it.
The Cheif releases Le Bon from jail when Dupin tells him the sailors story. He ends up selling the orangutan and Dupin is quite satisfied with himself and his ability to solve the case that no one else could.