Dupin is introduced. He lives with his friend in an apartment that they like to keep dark. Dupin is very analytical and solves crimes to test his knowledge.
It is discovered that Madame L'Espanaye and her daughter have been gruesomely murdered and stuffed up the chimney. The police do not know who the culprit is.
Dupin starts to investigate and finds that there was another source of entry for the culprit. A nail on the window was broken; therefore, the murderer entered through it. He deducted the murderer would have to be very strong and agile.
Dupin deducts that a monkey committed the crimes, because the hand prints on the daughters corpse and the strength it would require to shove both bodies up the chimney. Dupin also found animal hair in Madame L'Espanaye's hand. Dupin finds the monkey belonged to a sailor and places an add for the sailor to retrieve the monkey, which he does.
So what happened?
The sailor explains that the monkey was trying to shave. The sailor was afraid of the monkey having the razor and got the whip. The whip scared the monkey which made him panic and cut and strangle the victims.
After the mystery is solved, Le Bon is set free and the Monkey is sold and put in a zoo.