Devereux Bayne, a wealthy young American, resolves to fight with the French Aviation Corps during World War I. En route to Europe he meets the beautiful Esme Falconer and a man named Jenkins, who tells him that she is a German spy. Even though Esme will not divulge the facts of her mission to Devereux, he refuses to believe she is a spy and follows her to a small French inn. During the night, her chauffeur is killed, so Devereux takes his place and drives her to the Falconer chateau, followed closely by Jenkins and his men. There Jenkins reveals that he is really Von Blenheim, a German agent, and demands that Esme procure for him the plans which had been carried by her brother-in-law, a missing aviator known as "the Firefly." Esme and Devereux escape to the chamber in which the Firefly is hiding, and while the Frenchman and his sister escape, the American turns over a fake set of documents to Von Blenheim. The Germans drive Devereux towards enemy lines, but he signals the French forces and the spies are captured. He is wounded in the struggle, but Esme lovingly ministers to him until he is well.