Paris 1840: A Mother and Daughter are found dead in a locked room
The police arrest a courier who delivered coins to the women the day before their deaths.
Dupin, a book reading man who is obsessed with human behaviour, contacts the police and offers his expertise.
Dupin thoroughly examines the room where the women were attacked. He discovers strange hair left in the room. He also receives information from neighbours that there was shouting coming from the room, but it was in a foreign language none of them could understand.
Dupin places an advert in the local newspaper and is visited by a sailor. The sailor tells him that he brought an orangutan back from a recent voyage to Borneo and the orangutan had escaped.
The sailor sells the orangutan who commited the crime and the courier is released from prison.