It's 1776 and Benjamin Martin (MEL GIBSON) is an American colonist and recent widower trying to raise his seven children on his South Carolina estate. A veteran of the French and Indian War with a troubled past that still haunts him, Martin believes that King George's taxation without representation policy is wrong, but wants nothing to do with supporting a revolutionary war with the British.
Even so, a South Carolina assembly votes to have their state join the rebellion and Martin is unable to stop his oldest son, Gabriel (HEATH LEDGER) from enlisting. Hoping that his good friend and fellow war veteran Col. Harry Burwell (CHRIS COOPER) will watch over his son, Martin returns to his plantation and tries to get on with raising his family.
Yet, the forces under Gen. Cornwallis (TOM WILKINSON) soon take Charleston and the war looms closer. When a wounded Gabriel stumbles home and a battle literally breaks out in his front yard, Martin realizes he won't be able to escape the war. That becomes even more true when Col. William Tavington (JASON ISAACS) arrives and captures Gabriel, orders him to be hanged, and then shoots one of Martin's other sons as he tries to intervene.
Shocked and grieving over the sudden turn of events, Martin goes into warrior mode, and with the help of his two younger sons, ambushes a British unit and rescues Gabriel. Sending the rest of his kids to a plantation run by his sister-in-law, Charlotte Selton (JOELY RICHARDSON), Martin takes up arms against the British and is given command of the local militia. With the aid of French officer Jean Villeneuve (TCHEKY KARYO) who's been sent to teach the units about guerilla style warfare, Martin begins recruiting his old war buddies, such as John Billings (LEON RIPPY).
Meanwhile, Gabriel visits the hometown of his sweetheart, Anne Howard (LISA BRENNER), and gets various men to join the militia, including the town's priest, Rev. Oliver (RENE AUBERJONOIS). Rejoining forces, the father & son-led militia soon begin ambushing the British troops, singling out the officers to be the first targets.
When Martin briefly meets and then fools Cornwallis into releasing a group of rebel prisoners, the enraged general gives Tavington permission to do whatever it takes - no matter the moral implications - to find and stop Martin and his activities. With the tide continually turning in the war, Tavington then sets out to do just that, hoping to preempt Martin and his men's attempts at preventing Cornwallis and the British troops from moving north through the young country.