It is the late 18th century, in South Carolina. Benjamin Martin (Mel Gibson) is a veteran of the French and Indian War and a widower raising his seven children on his farm. Gabriel (Heath Ledger), the eldest, is anxious to join the American forces fighting the British in the Revolutionary War, even if it means doing so without his father's permission. Ben, who knows from first-hand experience the horrifying carnage that war presents from his experience in the French and Indian War, wants to discourage his son from participating.
When South Carolina votes to go to war, Gabriel joins the Continentals against his fathers wishes. He returns home after two years, stumbling wounded into the family home, carrying dispatches between commanders. That night, a skirmish between the British and the Continentals wakes the Martins and they give care to the wounded of both sides the next morning. British soldiers - the ruthless Green Dragoon cavalry - approach the house, proceed to kill the Colonial wounded, burn down the house and take Gabriel into custody as a spy, intending to hang him. Ben's 15-year-old (and second) son Thomas (Gregory Smith) is killed trying to free Gabriel as he is taken prisoner, shot by the ruthless and cold-hearted leader of the Green Dragoons, Col. William Tavington (Jason Isaacs) even though the boy posed no real threat.
An enraged Ben sets about to free his son Gabriel, with the help of his two younger sons Nathan and Samuel (played by Trevor Morgan and Bryan Chafin). The three of them manage to kill many of the British troops holding Gabriel (with Ben telling his boys to aim first for the officers). While their brother is freed, the boys are all horrified -- particularly Samuel -- at their first glimpse of their kindly father ferociously killing men with his knife and tomahawk. Gabriel re-joins the cause against his father's will again stating it is his duty as a soldier. Ben decides to join the fight later when he catches up with his son and they report together, leaving the rest of the children in the care of their aunt Charlotte (Joely Richardson), the sister of Ben's deceased wife.
Ben quickly concludes the poorly trained and ill-experienced Colonials cannot hope to beat the British in set piece battles - shooting at each other in open field, as was the custom of warfare at the time - because the British are too well trained and well armed. Continental Army Colonel Harry Burwell (Chris Cooper), having fought alongside Ben in the French and Indian War, asks Martin to organize a militia designed to keep British General Cornwallis in the south until the French navy arrives with 10,000 soldiers as re-enforcements. French Officer Jean Villeneuve (Tchéky Karyo), is present to help train the militia having witnessed his wife and daughters burned alive by the British army while aboard a ship.
The South Carolina militia is called up and adopts guerrilla tactics, using elements of surprise and attack, to harry the British supply lines (including the capture of Lord Cornwallis' personal effects and prize Great Dane's and the destruction of a supply ship in front of a ball at Middleton Place for the British officers). To combat the militia, Cornwallis - who had previously berated Tavington for his callousness towards civilians and executing surrendering troops, - authorizes Tavington to pursue more brutal tactics in order to draw Ben out. Tavington tracks Ben's family to their refuge with Charlotte and burns down her plantation. However, the family escapes, and are led to a safe haven by Gabriel. During this time, Gabriel marries Anne Howard (Lisa Brenner), a wartime marriage during a furlough. Soon after the marriage, returning home, Anne and her family, along with all the townspeople, are burned alive whilst locked in the church. The orders for this horrific act came from Tavington.
When a furious Gabriel discovers what has happened, he and a small group of men ride to engage the Dragoons. During the fight, many men on both sides are killed, leaving Gabriel and the Reverend to face off against Tavington. A few others escape with major wounds. The Reverend is shot, but throws his ready loaded musket to Gabriel, who shoots Tavington, who promptly falls to the ground. However, as Gabriel approaches Tavington's body with his knife in his hand to stab him, Tavington turns over and stabs Gabriel in the stomach with his sword. As Tavington escapes, Benjamin approaches the scene in time to find Gabriel dying on the ground. Ben is devastated and his zeal for combat extinguished. However Ben soon returns to the Continentals, carrying the American flag high, promising himself to stay the course.
In the final battle, Col. Harry Burwell and Villeneuve help Benjamin defeat the British, by using the militia (who are held in low regard by the British officers) as a feint. Although Col. Tavington has been told by Gen. Cornwallis not to intervene until ordered, Tavington and his Dragoons charge int