Anna Q. Nilsson is the title character, Agnes Lane, a daring spy for the South during the Civil War. She delivers an important message regarding an attack on a Yankee fort, then infiltrates the fort and turns herself in, only to change into a Union soldier's clothes and escape with more confidential information. The Confederate soldiers love her, and treat her with respect, despite her un-ladylike profession. During the attack, morale appears to flag, but she sneaks out a message that she has been captured by the Yankees and is due to be executed, urging the boys to greater heroism. As they capture the fort, she again infiltrates and pretends to be grateful to her "rescuers.�