In 1864, the Union army needs horses to mount an offensive against the South. The men at Fort Hedley in Colorado, led by Lt. Col. Hudson, have been unable to accomplish this mission because, despite all security precautions, raiders sympathetic to the Confederacy manage to steal them. As the raiders always know where the soldiers and horses will cross the mountain pass, Col. Sharpe suspects a southern spy is tipping them. When Hudson demands more men to fight off the raiders, Sharpe replies that no men can be spared, but shows him a new Springfield repeating rifle that will help his small troop become more effective. Later, a band of Union soldiers led by Major "Lex" Kearney, accompanies a band of horses across a desolate mountain pass, only to be met by a large party of raiders. Rather than risk the lives of his men, Kearney lets the raiders have the horses and leads the troop back to the fort. Kearney reports to Hudson in the presence of Matthew Quint, a private detective charged with discovering the southern spy. They are interrupted by Capt. Tennick, who demands that Kearney be court-martialed for cowardice in the face of the enemy and for sacrificing war materiel. Kearney, who is from Virginia, is suspected of southern sympathies and, despite the support of his men, is found guilty and ordered out of the military. Although he is prohibited from setting foot on an army base under penalty of death, Kearney stays nearby, taking every opportunity to taunt Tennick. One day, Kearney returns to his hotel to find his wife Erin waiting for him. She begs him to return home for the sake of his son Jamie, but Kearney refuses and sends her home alone. In the saloon, Quint informs Kearney that two of the raiders were captured during the army's latest encounter with them. Later, Kearney picks a fight with Tennick and during the brawl steps inside the fort. He is immediately arrested, sentenced to death and jailed along with the two raiders. Kearney soon helps the men escape and follows them to the ranch owned by Austin McCool, who has been selling horses to the Union army. After the two raiders vouch for Kearney, McCool shows him where he keeps the horses his men have stolen from the Union until they can be sold to the South. Kearney learns from the others that even his right-hand man, Pete Elm, does not know how McCool gets his information. After the horses are delivered to the South, Hudson, who is sympathetic to Kearney, finds him and tells him that Erin has returned and that Jamie has run away from school. Known only to Sharpe, Quint and Tennick, Kearney is actually a spy for the Union. Although he asks to be relieved of his assignment, so that he can join Erin in searching for Jamie, the others persuade him to remain on the job. Tennick agrees to kill McCool so that Kearney can take his place and subsequently learn who his contact is at the fort. Later, during a battle, Tennick wounds McCool, but is himself mortally wounded and dies just after firing the fatal shot to McCool. Back at the ranch, Kearney allies himself with Elm and waits for McCool's contact to approach them. To his surprise, the Union spy is revealed to be Hudson, who also discloses that a shipment of Springfield rifles, which he intends to steal for the South, will soon be delivered. When Kearney next reports to Sharpe, he learns that Jamie has been found. Delighted, he tells the news to Erin, who believes that he is lying in an effort to send her home again. She then visits Hudson, who confirms the news, but when Erin reveals that her husband told her, Hudson realizes that Kearney has been acting against him. Hudson then arranges for Sharpe and Quint to be killed, and later, Elm tells Kearney that Hudson will execute him as a southern spy. Before Kearney can be brought before a firing squad, however, the Union soldiers who served with him help him to escape. He tells them what he has learned, and they proceed to seize the new Springfield rifles, which they use to recapture the recently stolen herd of horses. During the ensuing fight, the raiders hide in the surrounding country, and Kearney's men must smoke them out. Finally, Hudson and the raiders are captured and Kearney is cleared. Witnessed by Erin and a proud Jamie, Kearney is returned to duty and becomes the head of the newly formed U.S. Army Intelligence Department. The efficacy of the Springfield rifle proven, it is now acquired by the Union army.