Ambler's novel is different from the movie on several counts, with the story narrated by Simpson (named Arthur "Abdel" Simpson in the book), so that the reader only gradually comes to work out what Harper and his associates are really up to. Simpson in the book is blackmailed into driving the car to Istanbul after Harper catches him trying to steal Harper's travelers' checks. The book features frequent flashbacks to Simpson's schooldays in England, which help to explain his character and motives more clearly than in the film.