from the back cover:
The room was on the fourth floor, and the door was locked - with the key on the inside. The windows were closed and fastened - on the inside. The chimney was too narrow for a cat to get through. So how did the murderer escape? And whose were the two angry voices heard by the neighbours as they ran up the stairs? Nobody in Paris could find any answers to this mystery.

Except Auguste Dupin, who could see further and think more clearly than other people. The answers to the mystery were all there, but only a clever man could see them.