On the last day of June, 1863, Harrison, a Confederate spy, is scouting the position of the Union Army in the vicinity of Gettysburg, Pennsylvania. Reaching the Confederate camp after nightfall, he reports the size and position of the enemy to General James Longstreet and General Robert E. Lee, who are surprised to learn that the enemy is so close. The generals decide to march toward Gettysburg at first light.
The next morning, in the Union camp, Colonel Joshua Chamberlain of the Twentieth Maine regiment is faced with a large group of exhausted soldiers who are refusing to fight. A rhetoric professor by vocation, he contemplates how to convey his ideals to the mutineers. He gives a speech explaining that the Union Army is unique within history; they are fighting to set other men free. Though he says he cannot force these men to fight, all but six join his regiment. Meanwhile, Union cavalryman John Buford scouts Gettysburg and claims Cemetery Hill, recognizing it as excellent ground for fighting.
That night in camp, Longstreet watches his men play poker with Fremantle, an English observer. George Pickett arrives with his brigade commanders, Garnett, Armistead, and Kemper. Longstreet and Armistead reminisce about Armistead’s friendship with Union Major General Winfield Scott Hancock and discuss Longstreet’s progressive—though, in Armistead’s view, impracticable—views on defensive warfare. Meanwhile, the other officers argue with Fremantle about the causes of the war. They are not fighting because of slavery, they insist; they are fighting for freedom from the rule of a foreign government.
On July 1, Lee wakes up feeling ill and troubled. There has been no word from Confederate scout Jeb Stuart, and Lee does not want any fighting until the army is all together in Gettysburg. As he and Longstreet ride together, they disagree—not for the last time—about the advisability of offensive versus defensive war. Lee believes that they should strike the enemy first and quickly, while Longstreet believes it is better to secure ground of one’s choosing and wait to be attacked. They hear artillery booming in the distance. Down the road in Gettysburg, Buford and his men are struggling to hold off some Confederate infantry. General John Reynolds arrives just in time to relieve Buford but is killed soon thereafter, and the Confederates score an early victory. As Chamberlain rides toward Gettysburg, he reflects on his youthful composition, “The Killer Angels,” a speech on the nature of man.
In camp that night, Longstreet speaks his mind to Fremantle, questioning the value of honor in war and describing the changing nature of tactics. But he soon gives up, finding that Fremantle, like Longstreet’s fellow Southern generals, is stuck in the past and considers honor more important than victory. Meanwhile, Lee confers with various generals and decides to attack the following day.
On July 2, while Chamberlain and his men wait to head into battle, Kilrain, his friend and fellow Union soldier, finds an injured black man in the woods. The man is an escaped slave who has been shot. Chamberlain sees to the man’s care and is startled and ashamed of his own reluctance to touch a black person. Later, he and Kilrain discuss their differing motives for fighting. Chamberlain believes in intrinsic human equality, while Kilrain is not an idealist; he is fighting to bring down aristocracy.
At Confederate headquarters, Lee lays out his case for attacking the Union position, to which Longstreet cannot agree. Nevertheless, Lee orders Longstreet to attack. As they ride toward the front lines, Lee tries to inspire Longstreet to be bolder in battle and reveals that he may not have long to live. Longstreet is depressed by Lee’s failure to understand his motivations. He is further grieved at having to send General Hood into battle, despite a poor position and the likelihood of heavy losses.
Chamberlain’s regiment is ordered to occupy a steep wooded hillside called Little Round Top. They are warned never to retreat, as they are the extreme left flank of the Union army. They succeed in holding off the Confederates for a while, but soon take heavy losses and run out of ammunition. Chamberlain has the idea to fix bayonets and take the Rebels at a disadvantage by charging downhill. They successfully overrun the Confederates and defend Little Round Top. However, Kilrain is shot twice and dies the following day.
After the battle, Longstreet is devastated by the grave injury of Hood and the losses of many others. He resolves to speak plainly to Lee; they cannot mount another attack. In Lee’s camp, however, there is an atmosphere of celebration. Longstreet is confused and silenced by Lee’s confident vision of victory. He later vents his frustration to Fremantle, explaining that there is no great strategy; Confederate victories have been fueled by blind love of Lee, not by sound tactics. If they win tomorrow, he says, it will be a miracle. Longstreet realizes he no longer idealizes Lee as a father figure. Lee, on the other hand, stays up late that night, offers a newly returned Jeb Stuart a mild reprimand for failing to keep him informed, and confirms his own decision to mount a fresh attack the following day.
The next morning, July 3, Longstreet finally manages to speak his mind to Lee, but Lee barely responds. Instead, he insists that the Union line will indeed break under a second attack, and, what’s more, he expects Longstreet to lead the charge. The Confederate artillery barrage begins later that morning. The officers have a variety of responses before the battle. George Pickett is overjoyed at the prospect of glorious combat; Dick Garnett resolves to redeem his honor, tarnished by perceived cowardice in previous battle. Armistead grieves the fact that he is about to go into combat against his friend Hancock. But all Longstreet can do is weep about sending men to probable death.
Sure enough, while a scattering of Confederates reaches the top of Cemetery Hill, most of the troops are slaughtered by the entrenched Union soldiers above them. Armistead and Garnett are killed. Pickett loses most of his officers. Longstreet is so horrified by the spectacle that he nearly walks into the field in hopes of being quickly killed. But he is stopped by the sight of Lee riding along the line, taking the blame for the disaster despite the protests of his adoring men. Longstreet does not want to speak to Lee, but when they finally meet, he accepts Lee’s humble admission of short-sightedness and promises to do what he can to help him in the future. However, he no longer believes the war can be won and doubts in his heart that he can fully forgive Lee. Together they make plans for the withdrawal of the army, under cover of an approaching storm.
That night Chamberlain sits on Cemetery Hill, overlooking the field of fallen soldiers. He is surprised by the beauty he has seen amidst the horror and doesn’t understand his own eagerness to return to the fight. Tom, his brother, joins him, and they discuss the Rebels’ courage on behalf of such a terrible cause as slavery. Chamberlain doesn’t understand it, but he cannot hate the Confederates, either. He thinks again of “the Killer Angels” and is thankful to have been part of this historic day. Finally, the long-threatened storm breaks, and the rain floods the fields of Gettysburg until the next day, which is the Fourth of July.