This 8th book in the "Outdoor Girls" series, find the girls and boys again at Pine Island, but under very much altered conditions. America had entered the great World War and all the boys but Will Ford have volunteered. Later the boys were called to Camp Liberty, some distance from Deepdale, and the girls conceived the plan of opening a Hostess House for the benefit of the relatives and friends of the boys.

While still at Pine Island the girls and boys had come upon a suspicious looking man in the woods Upon finding himself discovered the man had made his escape but in his hurry had dropped a letter which the girls found to their disgust was written in code They decided that the man must have been a German spy. (Adapted from the next book in the series)