Alvin York (Gary Cooper), a poor Tennessee hillbilly, is an exceptional marksman, but a ne'er-do-well prone to drinking and fighting, which doesn't make things any easier for his patient mother (Margaret Wycherly). He undergoes a religious awakening and turns his life around, assisted by Pastor Rosier Pile (Walter Brennan).
When York is drafted into the army for World War I, he tries to avoid induction as a conscientious objector due to his religious beliefs. His status as a true conscientious objector is rejected since his church has no official standing and he reluctantly reports for army basic training. During basic training, his superiors find out that he is a phenomenal marksman and promote him to corporal.
York still wants nothing to do with the army and killing. A sympathetic commanding officer lectures York about text from a U.S. history book. He gives York temporary leave to go home and think about fighting to save lives. York wants to read the U.S. history book and the officer gives it to him. He tells York that after his leave if he still doesn't want to fight he will discharge him from the army. York reads the book, decides he will serve his country and reports back for duty. York decides to leave it in God's hands, but still doubts he can kill someone because of his interpretation of the bible.
His unit is shipped out to Europe and participates in an attack. Pinned down by German fire and seeing his friends being shot down all around him, his self-doubt disappears. Owing to the large number of casualties, York suddenly finds himself placed in charge. He works his way around behind German lines and shoots with such deadly effect that the Germans surrender. Then, York forces a captured German officer (Charles Esmond) at gunpoint to order the Germans still fighting to surrender. He and the handful of other survivors end up with 132 prisoners. York becomes a national hero and is awarded the Medal of Honor.
York later explains that he did what he did to hasten the end of the war and minimize the killing.