George Cathcart, a young naval officer, arrives home on leave of absence after months of mine planting service. Major Maynard, a foreign spy, occupies the adjoining estate. With the assistance of Jessica, an adventuress, Maynard plans to obtain the maps in Cathcart's possession. Jessica contrives to become the guest of George's mother. Although he falls in love with her, the naval officer is led to suspect the girl. He removes the maps from his desk, hiding them behind a secret panel. When Jessica later rifles the desk, she obtains worthless maps. George has seen a yellow sunbonnet hanging in the hall of Maynard's home. The officer is later mystified when he finds a paper, lost by Jessica, bearing the legend, "the yellow sunbonnet." The two visit Maynard that evening. After they have gone, the spy unscrews the peg from which the sunbonnet hangs. The peg is hollow and contains the maps which Jessica had stolen. The adventuress has fallen madly in love with George and when informed by Maynard that the maps she had secured are worthless, refuses to aid him further. A clever ruse perpetrated by Maynard reveals to him the hiding place of the genuine maps. Jessica, however, makes this discovery first. When Maynard opens the secret panel, he finds a note in which Jessica tells him that she has the maps and, because of her love for George, intends to place them into the latter's hands. George comes upon the spy. Jessica enters the room just as Maynard attempts to shoot the naval officer. The girl leaps forward and receives the bullet intended for George. Maynard escapes. Jessica's wound proves slight and she becomes George's wife.