The plot consists of a number of episodes with little overall structure except connections to the coming German attack on France. An agent Mercier is running in Germany, a tank engineer lured by a sexual liaison, makes a narrow escape from the secret police. Mercier goes into Germany to observe secret tank exercises in the forest. And he extracts key evidence of German plans to invade through the Ardennes.
This seems a bit contrived, and Mercier himself is somewhat implausibly idealised. He comes from a minor aristocratic background, he was a fellow student of de Gaulle's, he played a key role in Pilsudski's turning of the Red Army flank in 1920, and so forth. And his romantic adventures start with a liaison with a princess and end with the finding of a wife (with some mildy gratuitous sex scenes). This all feels a bit artificial and Mercier never comes to life as a person.
Perhaps more importantly, there is no suspense in The Spies of Warsaw, not even an air of menace when Mercier is attacked by a group of Germans with a personal vendetta against him. The Spies of Warsaw worked just well enough to keep me occupied on a plane trip — the setting and the background details are convincingly depicted — but left me with no desire to read any more Furst.