By running a black slave on the run chased by Confederates, Corporal Blutch and Sergeant Chesterfield do not know that they will bring back to their base a strange character. Injured, he refuses to be examined by a doctor and turns out to be a white spy commissioned by McClellan to infiltrate enemy troops in order to report information about an imminent attack. Once the assault-surprise foiled, the newcomer (named Franklin Thompson) will be assigned to the medical service where he will assist the doctor and save many soldiers. Nevertheless, the secret continues to blind his true identity, and it will take all Blutch's insight to overcome the mystery...