An actress signs up to spy for the Union, then falls for a Confederate officer.

Early in the Civil War, the Second Battle of Bull Run is a disaster for the North. At a camp show for Union soldiers, performer Gail Loveless is recruited to become a spy by her friend, Pauline Cushman, who is herself a spy known as "Operator 27." Working for agent Major Allen Pinkerton, Gail agrees to become a spy known by the code name "Operator 13." She then goes South with Pauline to the headquarters of Confederate General "Jeb" Stuart. Posing as Pauline's black maid, Gail encounters Captain Jack Gailliard, a Confederate officer, when he rides by her washing and ruins it. Jack is a spy for the South, and when Pauline asks too many questions about him at a ball that evening, he and Captain Cornelius Channing become suspicious and have her room searched. Meanwhile, a traveling medicine show run by Doctor Hitchcock, who is secretly a captain in the Northern army, arrives looking for Operators 27 and 13. Gail is able to transfer information about Confederate troop movements to Hitchcock just as Pauline is being arrested. Gail is also suspected of being a spy, but when she is brought to testify at Pauline's trial, she divulges Pauline's real identity and says that her mistress "turned Yankee." Pauline is then sentenced to death, but Gail and Hitchcock help her to escape. Back in Washington, Pinkerton knows that Pauline can no longer operate across enemy lines, so he entrusts Gail with the mission to learn more about the activities of Jack, whom Pinkerton suspects is working with "Copperheads," Southern sympathizers, who live in the North. To make herself believable, Gail, using the name "Anne Claybourne," openly jeers at marching Union soldiers. She and a man posing as her father are then "arrested." When the incident is reported in Southern newspapers, "Anne" becomes a heroine and is deported to Richmond, where she becomes the guest of Mrs. Shackleford and her daughter Eleanor. Gail also re-encounters Jack, who is attracted to her, but suspects that he has seen her before. While at the Shackleford's, Gail is able to pass on information to the North that results in an important victory, but which causes the death of Eleanor's fiancé, John Pelham, just a few hours before their wedding. Feeling guilty over her part in John's death, Gail goes into the garden to cry and is met by Jack, who tells her that he loves her. Because she has also fallen in love with him, she moves him out of the aim of one of her operatives who is spying on them from the bushes. Soon, however, she gets away from Jack and, dressed in a Confederate soldier's uniform, heads North after the operative tells her that the Confederates now know she is a spy. As Jack and Channing chase her into the woods, they split up and Jack finds Gail asleep in a spring house. He then angrily calls her a a traitor and vows to take her back for a court-martial. He handcuffs Gail, but as they leave the house, they see Union soldiers execute Channing. Gail's operative then rushes toward the soldiers, but because of his rebel uniform, he is shot. The Union soldiers then decide to search the house, but leave after finding no one. Gail and Jack, who had hidden in the spring, now escape together and, after a blacksmith files their handcuffs off, Jack heads South, while Gail tearfully goes North. A few years later, the war is finally over and Gail and Jack reunite and pledge their love to each other.