Hannay experiences a shock when he returns to his apartment to find the corpse of the American Scudder with a long knife stabbed through his heart and pinning him to the floor. Hannay had allowed Scudder, a neighbor, to hide in his apartment after Scudder faked his own death by dressing up a corpse not unlike him to look even more like him.
When Hannay realizes that his apartment has been ransacked, he understands that the murderer must have been looking for something. He finds no trace of Scudder's little black pocket-book anywhere, so he assumes the murderer must have found it and taken it. Hannay had seen Scudder writing notes in this book and recognizes it must be important.
The next morning, however, shortly before he is about to flee, Hannay unexpectedly finds the black notebook when he sticks his fingers into the tobacco jar to fill his pipe. He takes it with him.
As Hannay is traveling, he pulls out the black book and studies it. He realizes it is written in code. He has some background in codebreaking, so he tries, but fails, to decipher it. Hannay has been missing adventure since his return from Africa, but now, he is deep into one.