During the American Civil War, Sergeant Barstow, a cynical, hard-boiled scout from Dakota, and Oliver Clark, an idealistic lieutenant from Boston, are both imprisoned in a Confederate prison camp on Belle Island. Oliver suggests that he and Barstow be assigned to care for a group of men infected with smallpox, and while they are being transported from Belle Island, the two escape. While hiding from their pursuers, they see Jenny, a Russian girl, kill the Confederate officer who betrayed her, and the pair offer her refuge. On the dead officer's body, they find a map written in Russian, detailing the positions of the Southern army for a massive ambush against the North, and realize that they must deliver the map to General Grant and his troops before the Confederate army has time to strike. Their mission is delayed when, while heading North, the trio stumbles across a murder in a farmhouse. They are at first trailed by the townsfolk, who accuse them of the murder, but are freed when their innocence is established. Soon after, however, they are trapped by Confederate soldiers. Splitting up, they make a getaway to a farm owned by Vestry, a Northern sympathizer. After they are driven from the farm by the advancing Confederate troops, the fugitives hide in the woods, where they watch as Grant's army rides toward the ambush. Oliver fails in an attempt to steal a horse to warn Grant, but Barstow, anxious to reach the Union lines to collect his back pay, takes the map and rides through a hail of gunfire in time to warn the Union army of the trap awaiting them.