A third attack on horseback approaches toward the fort at night, and more of the men perish. As the numbers dwindle, only twelve men are left. As the Sergeant stuffs more propped-up bodies into the parapets, the two Gestes comment on their commander's skillful tactics to outsmart the enemy:

John: Charming fellow, our Sergeant.
Beau: A trifle uncouth. But the best soldier we'll ever see.

The end looks near for the remaining survivors: "You'll all be with these others in a little while. You'll do your duty better dead than you ever did alive." During a break in the fighting, the two Geste brothers share a few final, fateful words together:

Beau: I remember ghastly Gussie said one time, 'The Gestes always stick together.'
John: Digby's probably on his way here now.
Beau: He'd better be. John, somebody always gets out of these things. It might be you.
John: 'The Gestes always stick together.'
Beau: Well, if they shouldn't, would you do me a favor?
John: Of course.
Beau: Thanks. In my coat, I have two letters in a packet. One letter in the packet goes to my Aunt Pat. The other letter stays here with me.
John: All right.
Beau: Could I do you a favor if, uh, things happen to be the other way around?
John: Just tell Isobel that, just tell her something.
Beau: Older brother takes care of everything, I promise you.

The embattled Markoff's strategy seems effective - the Arabs have retreated, and argue over whether to attack again: "They've lost their stomach for it. The next attack will be the last one. No sign of any relief from Tokotu. Well, we'll show 'em we're not only awake, but merry and bright!" The Sergeant orders the bugler to play, and then orders "some happy laughter - lots of it, seven is going to sound like seventy." During his jackal-like laugh, Rasinoff is shot and falls to his death from the tower, as another Arab attack commences. Markoff orders the few survivors to move from parapet to parapet as they fire to give the impression of greater strength and defense: "We'll make 'em think we're still fully manned."

Beau falls to a bullet, appearing to be dead as the Arabs retreat for the last time. Just in case they do return, Markoff props up a body in each parapet, and extols his own fortitude:

Look at 'em. They come when I want them and they go when I don't need them anymore. They're beaten! But they've put down a mutiny for me. They've given me the Legion of Honor. And they've made me an officer.

Only John and Markoff are left. John warns the commander not to touch Beau's body: "Leave my brother's body alone...If you touch him, I'll kill you." While John brings the Sergeant bread and wine from the barracks, Sgt. Markoff searches Beau's body and finds the sapphire and letters. In a confrontation when John returns and interrupts him, John pulls out his bayonet and Markoff draws his pistol and threatens: "Armed attack on a superior in the face of the enemy. I court-martial you myself. I find you guilty and I sentence you to death. You get six in the belly." With his last ounce of convulsive energy, Beau rolls over and upsets the feet and balance of the Sergeant, giving John the opportunity to bayonet Markoff in the heart with his sabre. Beau expires, in a gallant death scene, in John's arms:

Listen to me. I haven't got much breath. Leave the public letter in Markoff's hand. Take the packet and the other letter home to Aunt Pat...Go to Egypt. Promise?...Tell Digby I was sorry. I couldn't wait for him.

At that moment, the sound of a bugle from the relief column from Fort Tokotu under Beaujolais' command is heard outside the fort: "A lovely sound, but a little late." After hearing gunfire, John returns two shots from the parapet to allow time to escape - he fears that he will be court-martialed for the murder of the Sergeant. He places the public letter in Markoff's hand as instructed, and rests Beau's arms across his chest in peaceful repose. He retrieves a canteen and food from the barracks before jumping down the back wall of the fort and retreating into the desert.

When Digby scales the fort wall, he finds the body of his brother Beau but no evidence of John's demise. He pretends to be one of the dead soldiers when the Major arrives. While the Major searches around, Digby carries the bodies of Beau and Markoff into the barracks where he prepares the Viking's funeral for his brother. He sets up a funeral pyre, placing Markoff's body at Beau's feet [the symbolic dog at his feet], and then ignites the entire bier on fire. As it begins to burn, he plays his muted bugle in tribute, and then escapes over the back wall as he hears rifle shots diverting the relief column into the nearby oasis. He finds his brother John firing his rifle at the relief column. The two of them are reunited, and watch the "Viking's funeral" fire for their heroic brother as the fort is consumed in flames:

John: I knew you wouldn't forget.
Digby: He wouldn't have if it had been me.

As they speak about fleeing to Egypt, they meet their two legionnaire friends Hank Miller (Broderick Crawford) and Buddy McMonigal (Charles Barton) sent on horseback from the burning fort for reinforcements - they are given assistance in their flight from the Legion. Later, when the four of them encounter a band of nomadic Arabs at an oasis, Digby outsmarts the tribal band by giving the impression - with the blowing of a decoy bugle charge - that a larger Legion is attacking, but he is slain in the attempt. His body tumbles down the steep sand dune - his bugle rests half-covered by sand next to him.

John is the only brother who survives to return to England. At Brandon Abbas, he is reunited with his beloved Isobel and falls into her waiting arms. He also presents Lady Patricia with the letter from Beau, disclosing that the valuable gem had been sold years before, and that Beau had lifted the substitute to save embarrassment of Lady Patricia when she was instructed to sell it. She reads the letter aloud at the foot of the stairs:

My Dear Aunt Pat:
I was inside the suit of armor in the hall the day you sold the Blue Water to the Maharajah's agent and received an imitation to take its place. When the wire from Sir Hector came, I thought I could repay your devotion to us by giving Brandon Abbas its first robbery. So the lights went out and so did Beau. Lovingly, Beau Geste.

The final line is tearfully spoken by a grateful Lady Patricia after she finishes reading Beau's letter:

Beau Geste? Gallant gesture. We didn't name him wrongly, did we?

Beau's name aptly describes his gallant, beautiful gesture ('beau geste') for the entire family - he had repaid his Aunt Pat's devotion to them by sparing her humiliation over the jewel's sale.