During the French and Indian War frontiersmen aim to stop weapons being sold to Indians and have to challenge British forces.
In 1759 as English prisoners are exchanged, Captain Swanson (George Sanders) almost arrests Jim Smith (John Wayne) and the Professor (John F. Hamilton); but MacDougall (Wilfrid Lawson) gets them released. They meet Callender (Brian Donlevy) and learn he trades rum to Indians. Jim finds Janie MacDougall (Claire Trevor) grown up, and she expects to marry him. After an Indian attack, Jim takes ten men to fight them. Janie wants to go; but they are disguised as Indians without shirts. In a stream they capture an Indian and kill him after he talks. They go to Philadelphia to stop the selling of weapons to Indians, and Gov. John Penn prohibits it.
Callender ships powder and shot. Haughty Captain Swanson lets dangerous provisions go through; but Jim and his men go as Indians, and Janie wears a blanket. They attack the wagons and destroy the munitions. Callender burns the rest of the government property and reports to Swanson, whose soldiers capture Janie and four men. Jim and his men surround them to get the prisoners. Magistrate Duncan (Robert Barrat) promises they will be tried. Callender publishes a circular with Jim's name. Duncan suspects it and refuses to issue warrants. Duncan warns Jim that Gen. Gage is coming; but Jim decides to go after Callender. Men gather, and Jim gets a search warrant from Duncan, who holds a quick trial. Only Janie is convicted and locked up. Jim approaches the fort with a white flag, but Swanson refuses to let them inspect the goods. Jim is wounded and orders firing with rifles but no charge. Swanson keep his men awake. MacDougall goes for more powder, and Janie comes to wounded Jim. Swanson agrees to march out and leave the goods; but Callender and his men are detained and whipped. Jim sends the goods to Gen. Gage in Philadelphia. Janie with a rifle demands Jim see a doctor and takes him in a wagon.
Swanson returns to prosecute rioters, and Callender advises him. Several men are arrested. Will Anderson (Eddie Quillan) finds Jim and says many are in irons. Janie tries to stop Jim, but he kisses her. MacDougall and others pretend to be drunk; but Jim leads nine men at dawn and takes the fort to free the prisoners. Then Jim gives the fort back to Swanson. Callender kills Tom Calhoon (Moroni Olsen) but arrests Jim for it. From jail Jim tells MacDougall and a mob to disband. In the trial Callender says that Jim shot Tom from close range; but Duncan has Janie shoot a shirt from close and far, proving Tom was killed from a distance. Gen. Gage comes in, arrests Callender, releases Jim, and relieves Swanson of command. In the final scene Jim leaves for Tennessee, and Janie goes after him.
Friction between the American and British allies foreshadows the independence struggle. The Americans cling to principles of law despite British corruption and intransigence; but they are terrified of Indians.