Inside the Lines (UNABRIDGED‪)‬

"As World War I commences, a plucky young American woman in Europe on a business trip begins to realize that she may have remained too long. She and others anxious to leave for home are caught up in events swirling about them. A mysterious British officer becomes involved. This romance-tinged spy thriller begins on a train in Calais bound for Paris and ends in Gibraltar.
Based on a 1915 play of the same name, the novel was adapted for the screen in a 1918 silent version and again as a “talky” in 1930. This was the third novel written by Biggers, who went on to write six Charlie Chan novels in the 1920’s and 1930’s. -- Lee Smalley"