Charles Latimer, a mediocre English academic now enjoying success as a detective story writer takes a trip to Istanbul. There he meets a Captain of the local police who, as well as foisting his (painful) detective plot on Latimer, also tells him a tale of a real villain, Dimitrios, whose body has recently been fished out of the Bosphorous.
Intrigued by the story of the corpse, Latimer visits the morgue to view the body, and then (bored and listless) decides to dig into the history of Dimitrios, an enterprise that takes him to Sofia, Belgrade, Geneva, and finally Paris - where his amateurish investigations lead to disaster.
It's an unusual book - certainly not a spy story as we know it today, though there is an extended section about Dimitrios' activities as a spy in Albania between the wars. Told in a mixture of Latimer's present and flashbacks of his interviewees, Ambler's writing and style is terse and intelligent, and the detail he provides about the political and economic situation in Eastern Europe in the '20s and '30s convincing.