Ordered west following the Battle of Gettysburg, Union officer Samuel Wade reports to Major General William Rosecrans's Army of the Cumberland on the eve of the Battle of Chickamauga. Separated from friendly forces by the impact of the Confederate onslaught, Wade decides that the safest line of retreat is south, ultimately deep into enemy territory. Calling on skills he developed earlier in the war when he spied for the Union behind Rebel lines, Wade successfully passes as a Confederate officer on a special mission. Before he can safely make his way North, however, Wade discovers a conspiracy to assassinate President Lincoln! Driven by this knowledge, as well as the discovery that his brother Simon is incarcerated in Richmond's notorious Libby Prison, he is drawn back into clandestine operations behind enemy lines.