Two teenage step-brothers, newly qualified pilots, take their plane Her Highness on a trip to visit relatives and explore a border region and soon encounter a Canadian mounted policeman and a string of puzzling incidents. Their aerial patrols and on-the-ground inquiries uncover smuggling and other secrets around Isle La Motte, leading to rescues, captures, a dangerous tailspin and fire, and determined efforts to keep the mail and justice flowing. The narrative mixes aviation action, detective elements, and boys' adventure.