The film begins with a narrated map showing the Confederate States Army Army of Northern Virginia, commanded by Robert E. Lee, crossing the Potomac River to invade the North in June 1863, marching across Maryland and into Pennsylvania. On June 30, Confederate spy Henry Thomas Harrison reports to Lt. Gen. James Longstreet, commander of the First Corps, Army of Northern Virginia, that the Union Army Army of the Potomac is moving in their direction, and that Union commander Joseph Hooker has been replaced by George Meade. Longstreet reports the information to General Lee, who is concerned that the army is moving "on the word of an actor", as opposed to that of his cavalry chief, J. E. B. Stuart. Nonetheless, Lee orders the army to concentrate near the town of Gettysburg, Pennsylvania. At the Union encampments near Union Mills, Maryland, Col. Joshua Chamberlain of the 20th Maine Volunteer Infantry Regiment is ordered to take in 120 men from the disbanded 2nd Maine Volunteer Infantry Regiment who had resigned in protest, with permission to shoot any man who refuses to fight. Chamberlain speaks to the men, and is able to persuade all but six to take up arms.
Colonel Robert E. Lee resigns from the Union Army as the south secedes from the Union and both sides prepare for war. Major Jackson, who is a professor at Virginia Military Institute in Lexington at the outset of the war, leaves his family behind to do battle at Manassas Junction. Jackson is asked by a retreating General Barnard Elliott Bee, Jr. for assistance against the Federal army who is pursuing them after a brief stand on Matthews Hill. In rallying his shaken troops, Bee launches the name of Stonewall into history and the Confederates rout the Federals at Henry House Hill. Jackson maintains steadfast discipline in his ranks during the battle despite suffering a wound to his left hand from a spent ball.