It is autumn of 1861, in the midst of the American Civil War. A young Union Soldier named Carter Druse has been posted as a sentry near the edge of a cliff that overlooks a forested valley. In the valley hides five Union army regiments as they rest and prepare to surprise attack a nearby Confederate encampment. Although they are hidden in the valley, they are also vulnerable; there is only one narrow entrance and one narrow exit. The topography is such that they would be severely disadvantaged and unable to escape should their enemies discover them.
The narrator briefly recalls the morning that Druse left his childhood home. Druse is the son of wealthy Virginian parents and the product of a comfortable and cultured childhood. As a Virginian, it was expected that he would fight for the Confederate states. Despite this, his conscience compelled him to join a passing Union regiment. At the time Druse’s father called his son a traitor, yet also encouraged him to “do what you conceive to be your duty.” His father mentions that Druse’s mother is on her deathbed, but should both father and son survive the war, they may reconcile then. Both Druse and his father show respect for one another in their final parting, and Druse leaves his parents to become a Union soldier, proving himself both brave and noble.
The story returns to the scene on the cliff. Despite the importance of Druse’s keeping watch, and despite the fact that he is a noble and courageous man, he has fallen asleep. His rifle is already positioned to be fired, however, and Druse awakes to find that while he was sleeping, a lone horseman, a Confederate officer, has crept up to the cliff’s edge and is gazing down into the valley at the exposed Union soldiers. His face is turned so that Druse can not yet see his identity. The horseman is standing still and looks so picturesque and dignified in the afternoon sun that Druse is initially unsure if he is entirely awake. It seems to him that perhaps he has somehow slept until the end of the war and is now staring at a monument that has been erected in memoriam. He spends several moments admiring the splendor of the scene and the form of the horseman, which looks to him like a Grecian statue carved from marble.
The horse moves just enough to remind Druse of where he is and what he must do. He aims his rifle at the horseman’s breast. The horseman turns his head and seems to look straight at Druse, though he does not actually spy him in the bushes. Druse suddenly goes pale and nearly faints, overcome by the gravity of what he is about to do. He reflects on the ethics of killing an unaware man for the simple crime of possessing dangerous knowledge. He even briefly considers letting him wander on in the hopes that the horseman has not actually discovered the Union regiments, but quickly realizes this is a vain hope. Druse takes aim once more, this time at the horse. The words of his father echo in his head like a “divine mandate,” demanding that he put duty before all else. He steadies himself and fires. Both horse and rider fall over the cliff’s edge. The horse is killed by the bullet, the rider by the fall.
A wandering officer from the Union regiments has found himself standing beneath the cliff’s overhang. Looking upward, he sees the horseman still astride his horse, falling to his death. However, rather than perceiving what has actually happened, he mistakenly believes that it is a flying horseman. The vision seems to him an image of the apocalypse. He so sincerely believes that the horseman was actually riding through the air that when he hears the crash of bodies hitting the lower trees, he searches as if they had followed a gliding trajectory rather than falling straight downward.
After Druse fires, he reloads and remains lying in the shrub, keeping watch. His face is white, but beyond that he is unmoved. A sergeant crawls up to his position and asks what he has shot at. Without moving or looking at him, Druse reveals that the horseman he shot down had been his own father.