Amidst an ongoing guerilla war pro-slavery bushwackers raid the Bussey farm in Linn County, Kansas in spring 1861, leading 16-year-old Jefferson Davis Bussey and several friends of his to leave for Fort Leavenworth and enlist as a volunteer for the Union. Jeff takes a mutual dislike to a career officer, Captain Asa Clardy, whose abrasive personality make him both feared and hated by his men. Despite Clardy's vindictiveness, Jeff takes to military life.
Jeff's regiment marches off from Leavenworth to fight. Despite his eagerness, Jeff is repeatedly pulled away for various details and misses several battles. On one such occasion, Jeff resentfully confronts Clardy and taunts him, prompting a warning from fellow volunteer Noah Babbitt, an itinerant printer whom Jeff looks up to for his wisdom and years of experience. Jeff finally sees combat for the first time in the Battle of Prairie Grove; in the chaos following a charge against Confederate infantry, Noah and Jeff aid an undermanned Union artillery battery, firing a cannon as Confederate troops attempt to overrun the battery's position. Days later, Jeff and Noah receive the Medal of Honor for their valor during the battle.
During the campaigning in the contested Indian Territory, Jeff meets headstrong Cherokee girl Lucy Washbourne, whose well-to-do family is loyal to the Confederate cause. Despite their stark differences, Jeff and Lucy gradually grow fond of one another. Lucy's brother Lee is captured while spying on Jeff's regiment and Clardy orders that the boy be executed. Jeff refuses to join the firing squad and arranges for Lee's body to be returned to the Washbournes.
Impressed by Jeff's courage and ability, General James G. Blunt summons him to be assigned as a plainclothes scout. Going behind enemy lines in pairs, Blunt's scouts are assigned to gather intelligence regarding Confederate troop movements and report back. As they cross into Rebel territory in the dark, Jeff and fellow scout Bostwick blunder into a Confederate position and claim they are on their way to join the cavalry raider Stand Watie, only to find his force is camped right there. Left with no other choice, the two follow up on the lie and enlist as Confederate cavalry.
Jeff's name and straightforward manner lead him to develop mutual respect and trust with Watie's men. Jeff is called to join a guard for one of Watie's staff, who has been buying Spencer repeating rifles for Watie from a corrupt Union officer. As the latest shipment of rifles is sold, Jeff recognizes the officer as Clardy and realizes he must escape before Watie can buy enough Spencers to rearm his regiment, but abruptly falls ill and instead sends word ahead via Lemon Jones, an enslaved boy who agrees to escape and take Jeff's report on to General Blunt, hoping to join the U.S. Colored Troops when he gets there.
On leave in a nearby town one night, Jeff happens to meet Lucy and reluctantly tells her he is a spy for the Union. Despite her own loyalties, Lucy agrees to keep his secret. Returning to camp, Jeff stumbles into Clardy and is recognized. Clardy raises the alarm and Jeff goes on the run. Despite steadily growing weaker from lack of rest and food, Jeff keeps heading back toward Union lines as Watie's men chase him with the aid of Sully, a Texas bloodhound. Sully catches Jeff, but quickly takes a liking to him. Seizing on this opportunity, Jeff coaxes Sully into joining him, and the two leave Watie's men behind. Exhausted, Jeff reports in to a Union cavalry patrol and is overjoyed to find one of the troopers is Noah.
Noah reveals that Lemon Jones reached General Blunt, who stopped the supply of Spencer rifles to the enemy. Clardy's real identity is discovered by Watie's men, who take revenge for the execution of Lee Washbourne and rob him of his illicit gold. With the war over, Jeff's regiment disbands and he and his friends from Linn County return home. Lucy sends him a letter, prompting Jeff to decide he will soon propose to her. Returning to his family's farm, Jeff is offered a room but feels restless, instead finding peace outside, under the stars.
