Based on a true story, after getting married Williams becomes a moonshiner and goes to prison for a questionable murder. He is disciplined for being independent but invents a better repeating rifle.
Gun-designer David Marshall Williams (James Stewart) leaves the Winchester company to go home to his wife Maggie (Jean Hagen). He takes his son David to prison where Captain H. T. Peoples (Wendell Corey) tells David that his father killed a man.
Young Marsh Williams was in the navy and comes home. He tells his father (Carl Benton Reid) he is going to marry Maggie and wants his share of the farm, but his father says he has to wait two years. Marsh and Maggie get married.
Marsh works on a railroad for forty cents an hour but quits to distill liquor during Prohibition. He tells Maggie he got a raise. A moonshiner is killed by federal officers, and Marsh threatens them. Marsh is injured and tells Maggie about his still. Officers find the still, and in the shoot-out one is killed. Maggie suggests that Marsh give himself up, and he agrees.
In the trial it is unclear who killed the man, and the jury is deadlocked. Marsh pleads guilty to second-degree murder and is sentenced to thirty years. He says goodbye to his family and tells them not to write. Maggie has Marsh promise that he will not try to escape.
In prison Williams works, collapses, and his appendix is removed. Guards go after Dutch Kruger (Paul Stewart), and the stool pigeon is knifed. Kruger and Williams are put on the chain gang. A man is whipped, and they are transferred. Williams is chained to Kruger and takes a knife from him during an escape attempt.
Maggie writes to Williams, but he throws her letter away. Captain Peoples tells Williams to write her, and he lets him fix a gun. While working Williams takes Peoples’ pistol and kills a rattlesnake. Kruger has malaria, and Williams is late. He quarrels with Peoples, who sends him to the hole. After thirty days a doctor orders him let out of the hole.
Peoples shows letters to Williams and gives him 24 hours with Maggie. Williams tells her to get a divorce. Peoples lets Williams work in the blacksmith shop. Peoples sees Williams’ drawing of a gun. Williams explains what he learned in the hole.
Williams fixes a gun and works on his own gun. Kruger takes a gun from Williams and tries to escape and is killed. Peoples takes the other gun from Williams, who says he is working on a repeating rifle. Maggie reads a letter from Williams aloud to his family.
A newspaper reports that Williams will test his new gun, and Peoples is reprimanded by a board. Williams shows the board his gun, and Peoples persuades them to let him test his gun. A man from the Winchester company is interested. After eight years Williams is pardoned.
Peoples tells David that Williams has 68 patents, and eight million of his M-1 carbines were used in World War II.
This biopic rather accurately tells the story of an independent inventor who learned how to use his time in prison to be creative despite the harsh conditions.