Joshua Lawrence Chamberlain, a major character, remembers reciting to his father the speech from Hamlet: "What a piece of work is man...in action how like an angel!" Chamberlain's father comments, "Well, boy, if he's an angel, he's sure a murderin' angel."
In late June 1863, General Robert E. Lee leads his army into Pennsylvania. By threatening Washington, D.C., he hopes to draw the Union army into battle and inflict a crushing defeat, which will bring an end to the war. Harrison, a spy, tells General James Longstreet, Lee's friend, that the Union army is drawing near. They go to Lee, who is reluctant to trust a spy, but has to, because his usual source of intelligence, Jeb Stuart's cavalry, is out of touch. He sets out to meet the enemy.
At the road junction of Gettysburg, Confederate infantry encounters the Union cavalry of General John Buford who seizes the high ground and holds it against a Confederate attack at dawn on July 1. Troops of General John Reynolds come to support Buford. Reynolds is killed and the Union troops are pushed back, but at nightfall they entrench on high ground while the Confederates celebrate what appears to them to be another Lee victory.
Longstreet is filled with foreboding. On July 2, he tries to persuade Lee that the Union position is too strong. He urges Lee to march away and make the fight on more favorable ground. But Lee orders a flanking attack on the Union position. Colonel Joshua Lawrence Chamberlain of Maine is told by his superiors that he occupies the end of the Union line, and that he must hold at any cost. In a brilliant, costly action, Chamberlain succeeds in repulsing the Confederate attack.
On July 3, Lee orders a frontal assault on the center of the Union line. Knowing it is doomed, Longstreet argues against it. But Lee is confident and Longstreet despairs. General George Pickett leads the charge, which is turned back with heavy losses. A shaken Lee orders retreat. Chamberlain is now confident of Union victory.
Beginning with the famous section about Longstreet's spy Harrison gathering information about the movements and positions of the Federals, each day is told primarily from the perspectives of commanders of the two armies, including Robert E. Lee and James Longstreet for the Confederacy, and Joshua Lawrence Chamberlain and John Buford for the Union. Most chapters describe the emotion-laden decisions of these officers as they went into battle. Maps depicting the positioning of the troops as they went to battle, as they advanced, add to the sense of authenticity as decisions are made to advance and retreat with the armies. The author also uses the story of Gettysburg, the largest battle in the history of North America, to relate the causes of the Civil War and the motivations that led old friends to face each other on the battlefield.