The place is Venice, Italy. Richard Hannay assists seductive artist Dore Nicholson to board the Trans-European train to London, on which he is also a passenger. Other passengers include Boy Morland, Hannay's old friend, now a Diplomatic Courier, and Auguste Karrs, a Turkish Oil Tycoon.

Hannay and Dore Nicholson spend a romantic evening together. However, Dore is being watched by Conrad Smith, Karrs' personal secretary. Smith asks Boy Morland to use Hannay to get the information he wants from Dore.

Boy Morland warns Hannay, but Hannay does not take him seriously.

Why is Auguste Karrs so interested in Dore Nicholson's paintings, most particularly the ones that she painted in Mesopotamia? Dore refuses to show the paintings, but before going to bed, she secretly gives a notebook of her sketches to Hannay. The next morning she has disappeared.

What was the mysterious Frau Steve doing in Dore's compartment on that morning? Hannay decides to investigate, and is aided by Colonel Wormesl