Biggles, helped by his erstwhile enemy Eric Von Stalhein, sets off to behind the Iron Curtain in a bid to rescue Biggles' one true love, Marie Janis.

goodreads summary

Biggles has dinner with old enemy, and now friend, Erich von Stalhein, who asks him if he ever wonders what happened to Marie Janis, a young girl Biggles fell in love with during the First World War. (This tale is told in a short story called "Affair De Coeur" in the first ever Biggles book THE CAMELS ARE COMING). Biggles is astonished to hear that Von Stalhein has had a letter from Marie. She is in Czechoslovakia and the letter contains clues that all is not well. Later, Von Stalhein disappears and Biggles concludes he has gone to rescue Marie. A postcard from Rodnitz with a cryptic message confirms this.