The film opens with old war artifacts being loaded into a trunk, and a voice over stating that some memories of the past are best kept hidden away in like manner. It's clear that these memories still haunt our hero, Benjamin Martin (Mel Gibson), but you fear that these memories will not be mentioned, that we will just have to take his word for it. As the film progresses, we learn that Benjamin Martin has quite a reputation, earned in the French and Indian war, but he has kept his children in the dark regarding his past war experience. This sets up the first act, with the rumor of all out war with the British, Benjamin's reluctance to get involved in another war, and his son's eagerness to join up. By keeping his past a secret from his children, he has no one but himself to blame when his eldest son, Gabriel (Heath Ledger), joins the newly sanctioned South Carolina army ready to take on the world.

We see bits and pieces of the war through letters and flashes Gabriel writes to his family. So, a year goes by, when suddenly, an injured Gabriel stumbles into the family home, bearing urgent documents for the local military command. War is raging all around the countryside, and in the morning, the Martin family is seen taking care of the wounded of both sides. The British army arrives, thanks Benjamin, and begins to gather their wounded, when the Dragoon unit arrives. Their leader, Colonel Tavington (Jason Isaacs) is as ruthless as he is a brilliant officer. In quick order, he has ordered the Martin estate burned, the horses garnered for the British armies' use, the supposed slaves freed and conscripted, and had Gabriel arrested as a spy, set for execution. Benjamin tries to intervene, stating the rules of engagement, and fails. His second son, Thomas, then charges in and tries to free Gabriel, only to be shot in the back by Tavington. Gabriel is still taken away, Benjamin is an emotional wreck, and the farmhouse is burning.

The stage has now been set for Benjamin to enter the war in a big way. He charges into his house to retrieve his rifles, and his relic tomahawk from the previous war. He tells his young children to hid in the woods, while he grabs his two middle sons, and they quickly follow the British unit taking Gabriel to his execution. A lightening ambush results in all but one British officer dead, Gabriel free, Benjamin bathed in blood, and the two younger boys tramatized. He packs off his younger children to their aunt Charlotte's (Joely Richardson) house, and both Benjaming and Gabriel rejoin the main force of the South Carolina army. It is here that we first meet Colonel Burwell (Chris Cooper) and the French liason to the revolutionary army, Jean Villeneuve (Tcheky Karyo). Benjamin is immediately awared a field commission, and sets off to recruit men for his militia unit.

The next several months pass by with Benjamin's militia unit hijacking and ambushing British convoys and units left and right. General Cornwalis (Tom Wilkinson) is furious when one of those shipments turns out to be his personal belongings, and lights a fire under Tavington to do what it takes to bring in this militia. During this time, a romance buds between Gabriel and a local girl, and they end up married. It's not long before Tavington's forces begin to fight fire with fire, and capture a portion of the militia unit. Benjamin is forced to sue for a trade of hostages, which ends up being a ruse, and only makes matters worse by making a fool of Cornwalis. The gloves have come off now, and Tavington goes after the families, starting with Charlotte and Benjamin's younger children. When this fails, he extends his reach to include as many families as he can find, including the entire village where Gabriel's new wife lives. After questioning the villagers, Tavington has them locked in a church and sets the whole place on fire.

When the militia finds this, and realizes what has happened, Gabriel and a few men take off after Tavington with revenge in mind. They quickly catch up to them and a fight ensues with Gabriel winding up dead, and Tavington shot, but able to escape. Benjamin is so distraught over this, he is ready to pack it all in and hide from the rest of the war. It is only finding the old, worn out flag that Gabriel had been trying to mend, that he realizes how much his son truly believed in helping to build a new republic. Benjamin rides into camp with this new hope, and the stage is now set for the final big battle.

Benjamin persuades the Army to trust his militia to draw the British in for a large scale ambush of sorts. Not only are the British regulars drawn into this trap, but Tavington's Dragoons are pulled in by his own need for closure to this personal battle. Just as the Revolutionary army is wavering, Benjamin rallies them and draws the attention of Tavington. Amidst all the fighting taking place all over the great field, Tavington and Benjamin have their final battle. As Benjamin defeats Tavington, Cornwalis realizes he has lost this battle. This turns out to be one of the decisive battles of the Revolutionary war. The film is bookmarked with an encapsulated voice over stating how the tide was turned, and that Cornwalis was finally defeated once and for all at Yorktown, when the French finally showed up. We end with a view of Benjamin, his children, and Charlotte pulling up to see their neighbors building him a new home.