In the summer of 1940, the men of a National Guard unit from Green Hill, Oklahoma enjoy the camaraderie of their annual two-week maneuvers, which culminate in a town dance. At the end of summer, when Congress calls up the National Guard, the men are sent to Ft. Sill, and later, Camp Barkeley. Two of the Guardsmen, best friends Tom McCreery and Gil Hackett, leave behind Mary Caldwell, the girl whom they both love but who cannot choose between them. As soon as they arrive, Durkee, a tough-talking Army sergeant, complains about their greenness and bullies their own Sgt. Braggart in front of them. When Durkee insults their Thunderbird insignia, Tom fights him until Sgt. Logan breaks them up and puts Durkee in his place. While the Green Hill men retake basic training with new draftees, Tom tries to tell the mysteriously unresponsive Logan about his father, a West Point-trained lieutenant who died a hero's death during World War I. When the Japanese attack Pearl Harbor, the Green Hill men are sent overseas. On the ship, the spoiled banker's son from Green Hill, Keith Watson, taunts Tom with a rumor that Tom's father was dishonorably discharged, and Logan, who overhears them, interrupts the argument. After the men endure their first combat, which is a frightening experience unlike their maneuvers, Tom describes it in a letter to Mary, including how Keith was the first of them to capture prisoners. At home, Mary, having come to a decision, sends a recording to Tom and Gil, explaining that she is in love with Tom. Gil tries to hide his heartbreak, but soon takes up with a pretty nurse, Lt. Ellen Henderson. The men fight in Sicily and Italy, and see their companions die, including Cpl. Ray Hanford, whose son was recently born. Keith, who has matured during combat, turns down a safe clerical position to stay with his unit and is sorry to have gossiped about Tom's father. However, Tom has learned from his uncle the truth about the rumor and is too ashamed to listen to Logan's attempt to comfort him. Later, Tom gets separated from the unit and is presumed dead, but Logan, in spite of Gil's insistance, will not allow a search. After several more battles, Logan leads a patrol to scout for Germans near Anzio. Leaving the patrol behind, Logan climbs a hill alone and, seeing that he is about to be shot, reports the approach of Panzers. His report saves countless American lives after planes are sent to stop them. After Logan's death, Gil is told that Logan left a letter explaining how he had been court-martialed out of the Army during World War I after disobeying orders to search for a friend, who was dead. The search alerted the enemy to the unit's location and resulted in ten more deaths. A West Point man, Logan felt lost without the service, so signed up again under an assumed name and had hoped to someday pass his bars to his son, Tom. After they are shipped to Normandy, Gil receives another recording, this time from Tom, who was wounded but now is fighting in a unit with two other Green Hill men. Meanwhile, the youngest member of the group, Calvin Jones, is killed by a sniper while shaving off his first whiskers. Weary of combat, Gil's unit crosses the Rhine. In a hostile German village, Braggart dies heroically while saving others from a grenade, and the tough Durkee, who has enjoyed a friendly antagonism with Braggart since Ft. Sill, is moved to tears. Gil and Tom are sent with their units on a rendezvous mission to bottleneck the Germans, keeping radio contact with Osage tribesmen speaking in their native language, which the Germans cannot decipher. After connecting, Gil tells Tom about his father, who died the hero's death that Tom was told about in childhood. When the war is finally won, the surviving Thunderbirds return home, and Gil and Tom reunite with their respective sweethearts, Ellen and Mary.