During the Korean War, a beleagured American Army division is forced to retreat, and the general in charge decides to station a 48-man platoon in a strategic mountain pass to prevent the enemy from learning about the move until after it is completed. The "rearguard action" is headed by Lt. Gibbs and sergeants Rock and Lonergan, who march their men toward the pass. During the march, Corp. Denno, a well-trained yet fearful soldier, is unable to kill a single enemy, despite past successes in shooting at a large number of oncoming men. Upon reaching the pass, the soldiers plan their fortifications, including laying a mine field on the pass ground and establishing lookout posts. The men bemoan the lack of dry socks and warm food, while Denno converses with Rock, who cautions him that despite his fear of leadership, there are only three men above him, and he will have to take command of the platoon if they are killed. The enemy attacks but the platoon responds quickly, and discovers a cave large enough for all the men to take cover in. Soon after, Gibbs is killed by a sniper, and Denno is haunted by the thought that now only two men stand between him and command. That night, Denno confesses to Rock that when he was in officers' training school, he twice gave unsafe orders that resulted in serious injuries to his men. Rock reassures Denno that he will be able to cope and advises him that the only thing he should rely on is his gun. In the morning, the men are awakened by the loud bugle calls of the enemy, and Whitey, a soldier christened "Mr. Belvedere" by his comrades because of his know-it-all attitude, explains that the Chinese are attempting to cause the Americans psychological distress. Rock sends Whitey and another soldier to steal one of the bugles, and although they succeed, Whitey's companion is injured. As the medic tends to the wounded man, Lonergan searches for soldier Bigmouth, who was not at his post. Lonergan soon finds Bigmouth, who has slipped into unconsciousness due to the cold, but as he is bringing him back, Lonergan is shot by a sniper. When the medic tries to cross the minefield to rescue Lonergan, he steps on a mine and is killed. Desperate to save Lonergan and avoid coming closer to command, Denno risks his life to traverse the minefield and retrieve the sergeant, but Lonergan is dead by the time Denno carries him back. The next day, the Chinese ambush one of the lookout posts and force the rest of the Americans into hiding in the cave. With only an hour to go before they can follow the main company and retreat, the men grow edgy and worry about ricochets within the cave. Their fear is realized when Rock is hit and killed by a sniper's ricocheting bullet, and Denno must face his worst fears and assume command of the platoon. Denno orders the men to wait until the appointed hour, but as they are about to decamp, they hear an approaching tank. Realizing that the enemy has figured out the rearguard strategy, Denno decides to blow up the tank in the pass so that the Chinese cannot follow the retreating troops. The men succeed in blowing up the tank with a bazooka, but are soon embroiled in a major battle with the enemy. Meanwhile, the main division succeeds in their retreat and blows up the bridge leading to their new position. As the soldiers set up camp, they see some men swimming across the river. Holding their fire, they recognize the battle-tested Denno and his remaining men, who have triumphed over their opponents. With his confidence restored, Denno remembers Rock telling him, "You ain't a corporal for nothing, corporal."