Based on the novel by P. C. Wren and the 1926 film, after a mysterious jewel theft three brothers join the foreign legion under a sadistic sergeant.
Major Henri de Beaujolais (James Stephenson) and his troops relieve a fort guarded by dead men, and he finds a note confessing to a sapphire theft.
Fifteen years earlier in England kid brothers Beau (Donald O'Connor), John, and Digby play navy with Isabel and burn a Viking funeral. Beaujolais meets the three orphaned brothers raised by Lady Patricia Brandon (Heather Thatcher). Fifteen years later Isobel Rivers (Susan Hayward) plays piano for John Geste (Ray Milland). Beau Geste (Gary Cooper) and Digby Geste (Robert Preston) refrain from killing a mouse. Patricia brings out the sapphire, and in the dark it disappears. Digby searches unpopular Augustus Brandon (G. P. Huntley). Digby shows John a note from Beau saying he took it. John gets a note from Digby that he did it. John says good-bye to Isobel.
At Saida harsh Sergeant Markoff (Brian Donlevy) orders new recruits and is warned by an officer. John joins Beau and Digby. Rasinoff (J. Carrol Naish) overhears the three and tries to steal the jewel but is caught. Markoff questions him and plans to take it from Beau. Markoff sends Digby away for training.
At Fort Zinderneuf Lt. Martin (Harvey Stephens) is ill, and he orders Markoff to hold two deserters for court martial, but Markoff drives them back into the desert. Lt. Martin tells Markoff to send for an officer and then dies. Markoff promises severe discipline. Beau and John oppose the mutiny led by Schwartz (Albert Dekker). Voisin (Harold Huber) tells Markoff of the mutiny. Markoff and Rasinoff disarm the sentry, and Markoff has Beau, John, Maris, and Voisin get the rifles. Markoff asks Beau for the jewel, but Beau says he doesn't have it. Markoff orders Beau and John to shoot Schwartz and Renoir, but they refuse. Arabs attack, and Markoff sends for help. Men are given rifles to shoot at Arabs, who retreat. Markoff sends Schwartz up on the tower. Arabs attack again. Markoff places dead men on the wall and sends Voisin to the tower. Arabs attack at night and retreat again. Markoff sends Rasinoff to the tower. In case of death Beau asks John to send a packet to Patricia, or he will take care of Isobel.
Markoff makes the last seven men laugh. Rasinoff is killed. Beau is shot. Markoff props up the dead and finds the jewel on Beau and is about to shoot John when Markoff is killed. Before dying, Beau tells John what to do. John puts the note in Markoff's hand and escapes as bugler Digby climbs into the fort. Digby hides as Beaujolais enters and finds the note. Digby carries Beau's body to his bed and puts Markoff's body at his feet, then burns the barracks, playing taps. As Arabs attack troops at the oasis, Digby escapes to John; they are joined by Americans Hank Miller (Broderick Crawford) and his buddy. Digby plays his bugle and is shot.
John returns to Isobel and Patricia, who reads the letter from Beau saying he stole the imitation, because he saw her sell the sapphire.
This improbable story glamorizes a sacrificial gesture
that results in soldiers, buoyed by brotherly comraderie, suffering
under a sadistic officer before being killed by Arabs in the desert.