In 1860, Captain John Nelson, a U. S. Army officer working undercover as a civilian, is assigned to aid in the construction of the vital east-to-west Kansas-Pacific Railroad, on which the work is being continually delayed by sabotage from an unknown group of sympathizer with the Southern cause. He finally succeeds in weeding out the infiltrating souther-workers, but the latter then wage a small-scale war against the railroad builders, which climaxes in a last desperate attack on an armed-train.